اکینودرماتا-Echinodermata
Echinodermata
A phylum of exclusively marine animals with
a peculiar body architecture dominated by a five-part radial symmetry.
Echinodermata [from the Latin echinus (spine) + dermis (skin: “spiny skins”)]
include the sea stars, sea urchins, and related animals. The body wall contains
an endoskeleton of numerous plates (ossicles) composed of calcium carbonate in
the form of calcite and frequently supporting spines. The plates may be tightly
interlocked or loosely associated. The spines may protrude through the outer
epithelium and are often used for defense. The skeletal plates of the body wall,
together with their closely associated connective tissues and muscles, form a
tough and sometimes rigid test (hard shell) which encloses the large coelom. A
unique water-vascular system is involved in locomotion, respiration, food
gathering, and sensory perception. This system is evident outside the body as
five rows of fluid-filled tube feet. Within the body wall lie the ducts and
fluid reservoirs necessary to protract and retract the tube feet by hydrostatic
pressure. The nervous system of these headless animals arises from the embryonic
ectoderm and consists of a ring around the mouth with connecting nerve cords
associated with the rows of tube feet. There may also be diffuse nerve plexuses,
with light-sensing organs, lying below the outer epithelium. The coelom houses
the alimentary canal and associated organs and, in most groups, the reproductive
organs. The body may be essentially star-shaped or globoid. The five rows of
tube feet define areas known as ambulacra, ambs, or radii; areas of the body
between the rows of tube feet are interambulacra, interambs, or interradii.
The larvae are usually planktonic
(free-floating) with a bilateral symmetry, but the adults are usually sedentary
and benthic (bottom-dwelling in a marine environment). They inhabit all oceans,
ranging from the shores to the greatest ocean depths.
The phylum comprises about 7000 existing
species. The Echinodermata have a good fossil record with about 13,000 fossil
species. They first appeared in the Early Cambrian and have been evolving, since
pre-Cambrian time, for well over 600 million years. During this time several
differing body plans have arisen. The surviving groups show few resemblances to
the original stock. The existing representatives fall into five, possibly six,
classes: Crinoidea (sea lilies and feather stars); Asteroidea (sea stars);
Ophiuroidea (brittle stars); Echinoidea (sea urchins, sand dollars, and heart
urchins); and Holothuroidea (sea cucumbers). The recently described class
Concentricycloidea (sea daisies) has been referred to the Asteroidea by some
experts and retained as a distinct class by others.
The outline of classification for the phylum
is shown below. Classes with living representatives are elaborated to the level
of subclass or order.
Phylum Echinodermata
“Homalozoans”
Class: Ctenocystoidea
Stylophora
Homostelea
Homoiostelea
“Crinozoans”
Class: Eocrinoidea
Paracrinoidea
Rhombifera
Diploporita
Crinoidea
Blastoidea
Coronoidea
Parablastoidea
Edrioblastoidea
Subclass: Inadunata
Camerata
Flexibilia
Articulata
Order: Millericrinida
Cyrtocrinida
Bourgeticrinida
Isocrinida
Uintacrinida
Roveacrinida
Comatulida
“Asterozoans”
Class: Somasteroidea
Asteroidea
Order: Platyasterida
Trichasteropsida
Paxillosida
Notomyotida
Spinulosida
Valvatida
Velatida
Forcipulatida
Class: Concentricycloidea
Order: Peripodida
Class: Ophiuroidea
Order: Stenurida
Oegophiurida
Phrynophiurida
Ophiurida
“Echinozoans”
Class: Helicoplacoidea
Camptostromatoidea
Edrioasteroidea
Cyclocystoidea
Ophiocistioidea
Echinoidea
Subclass: Perischoechinoidea
Order: Bothriocidaroida
Echinocystitoida
Subclass: Cidaroidea
Order: Cidaroida
Subclass: Euechinoidea
Order: Diadematoida
Echinothurioida
Pedinoida
Arbacioida
Echinoida
Phymosomatoida
Salenioida
Temnopleuroida
Clypeasteroida
Holectypoida
Cassiduloida
Spatangoida
Class: Holothuroidea
Order: Dendrochirotida
Dactylochirotida
Aspidochirotida
Elasipodida
Molpadiida
Apodida
Morphology
Despite displaying an astonishing variety of
body forms, existing echinoderms share common anatomical features: symmetry,
body wall, skeleton, nervous system, coelom, alimentary system, reproductive
organs, and water-vascular system.
Symmetry
Adult echinoderms usually show pentamerism,
a five-part radial symmetry in which there are normally five axes radiating out
from a central point in the body. The mouth may lie on the underside, and the
anus may lie on top of the animal. There are numerous exceptions: in the
crinoids the mouth and anus lie on the same side, and in the holothuroids the
mouth and anus lie at opposite ends of a cylindrical body. The side of the
animal bearing the mouth is termed the oral side, and the side away from the
mouth is the aboral side. The relationship of the mouth with the substratum was
at one time considered important so that those echinoderms which were
essentially sessile, with a mouth facing away from the substratum, were
classified as subphylum Pelmatozoa, while those mobile forms in which the mouth
faced the substrate were classified as Eleutherozoa. These differing life habits
have little bearing upon formal echinoderm classification, but the terms
“pelmatozoan” and “eleutherozoan” are useful descriptors of lifestyles in the
phylum.
It has been suggested that five-part
symmetry in echinoderms confers strength upon the skeleton supporting the body
wall. Pentamerism was not an original feature of the echinoderms. Certain early
groups such as the carpoids did not show it, and modern echinoderm larvae have
bilateral symmetry.
Body
wall
The body wall is cloaked in a thin
epidermis, a layer of epithelial cells. The external layer is often ciliated,
and the electron microscope has shown that these cells have an outer membrane
that bears a great number of minute tubular extensions like microvilli. This
arrangement of the epidermis is similar in appearance to the absorptive
epithelium of many intestinal tissues. Below the epidermis, the dermis contains
the skeletal elements. Internal to the dermis are the body-wall muscles and
connective tissue. In the crinoids, ophiuroids, some asteroids, and some
echinoids, the skeletal elements make up a great part of the body-wall volume.
In the more flexible asteroids, the spherical echinoids, and the holothuroids,
the volume of these elements is reduced. The inner surface of the body wall is
also lined with epithelium and borders the coelom.
Skeleton
The skeleton is unique and consistent in all
groups. Almost every plate, spine, or ossicle is composed of a single calcite
crystal. During development each plate grows as a result of calcite secretion by
a group of cells. The plate grows like a three-dimensional lattice (stereom) to
produce a reticulate (having or resembling a network of fibers or lines)
ossicle, with the tissue that secretes and maintains it (stroma) occupying the
spaces within (Fig. 1).
Fig. 1 Cross section through the spine of an
echinoid showing the microscopic structure of the skeleton. The stereom forms a
continuous mesh, and the stroma that secretes it lies in the interspaces which,
in this case, are arranged in radial rows.
Sensory and neuromuscular
system
Echinoderms have simple sensory systems. The
nervous system is ectodermal in origin and comprises a radial nerve cord lying
along each ambulacrum. Each radial nerve connects with the circumesophageal
nerve ring, and it is believed that interradial coordination takes place via
this ring. The absence of a head means that sensory receptors and large areas of
integrative nervous elements are not aggregated in one place. The nervous system
is essentially diffuse, although specialized sense organs, such as the recently
discovered “eyes” of certain brittle stars, occur. It appears that the
circumoral ring and radial nerve cords take responsibility for gross activites
such as posture, coordinated locomotion, and food collecting, but in the
asteroids and the echinoids there are well-developed basiepithelial nerve
networks that coordinate the numerous test appendages. Echinoderm muscles are
principally smooth, but striated muscles have been detected in a few
instances. See also: Nervous system
(invertebrate)
Coelom
The coelom is extensive and usually arises
by enterocoely, or pouching, from the gut; but when there is direct development,
with larval stages greatly reduced or absent, it may be schizocoelic, involving
splitting in the mesoderm. The perivisceral coelom surrounds the viscera. During
development, part of the coelom forms the water-vascular system, so the spaces
within the tube feet and vessels are coelomic. See also: Coelom
Alimentary
system
In general the mouth is situated on the same
side as the tube feet. From it arises the alimentary canal, which runs through
the body to the anus (absent in the ophiuroids and some asteroids). The gut is
lined by endoderm, and in some groups digestive caeea (blind pouches at the
beginning of the large intestine) increase the area for absorption.
Particulate food is collected by the tube
feet in many crinoids, ophiuroids, and holothuroids. Most asteroids are
carnivores and engulf the prey, or evert part of the stomach over it, while some
echinoids have well-developed chewing teeth for rasping at encrusting plants and
animals. Many holothuroids, some echinoids, and a few asteroids ingest mud or
sand, obtaining sustenance from associated organic material such as bacteria and
diatoms. See also: Digestion
(invertebrate)
Reproductive
system
The reproductive organs lie within the
coelom in the interradial position, except in the crinoids in which they arise
on the arms. In most echinoderms there are five compact gonads, but the
holothuroids have just one, and certain echinoids may have two, three, or four.
The sexes are usually separate, The gonads discharge gametes by short ducts into
the surrounding seawater. In some echinoderms, especially in polar regions,
fertilized gametes are retained, and the young develop in special sacs in the
body wall.
Water-vascular
system
This is a multifunctional fluid-filled
coelomic system that probably evolved first as a respiratory system but later
took on increasingly important roles in food collection and locomotion (Fig. 2).
It comprises a number of protrusible, hollow, tentacle-like tube feet, arranged
externally in rows along each radial area of the body. These areas, composed of
single to multiple rows of tube feet, are termed ambulacra. A small modified
test ossicle, the madreporite, perforated by many irregularly shaped pores,
connects with the water-vascular system via a calcified stone canal. It was
thought that the water-vascular system opened directly to the surrounding
seawater via the madreporite so that extra fluid could be drawn in to replenish
the water-vascular fluid. The fluid volume of the water-vascular system may be
more constant than was previously believed, and relatively little fluid passes
in or out of the madreporite. Each tube foot is associated with a small bulbous
reservoir inside the body. The feet are extended hydrostatically, and retracted
by longitudinal muscles in their walls.
Fig. 2 Diagram of the water-vascular system in
an echinoid. Arrows show direction of fluid
flow.
The form and the activity of tube feet vary
greatly between the groups of echinoderms. In the crinoids they are fine,
tapering structures suited for food collection and having no locomotory role. In
the brittle stars they lack terminal suckers and are more important for food
gathering than locomotion. In some asteroids, particularly the burrowers, they
are tapering and pointed, while in others they are suckered and capable of
adhering to the substratum. This is also the case in regular echinoids (which
have no front or back end and can move in any direction), but in irregular
echinoids (which have a definite front and back and do move in a particular
direction) the tube feet are modified to suit the burrowing way of life. In
holothuroids the tube feet surrounding the mouth are modified as tentacles of
varying form and complexity and are used for food collecting. The rest may be
present as locomotor or respiratory organs. In a few groups, such as the apodous
and molpadiid holothurians, tube feet are missing from the body wall. When tube
feet serve an important locomotor function, postural muscles govern the stepping
movements.
Each tube foot is under the control of the
radial nerve cord and is connected by a short lateral branch to the radial
water-vascular canal as well as to the individual ampullae. All the radial
canals are linked by the circumoral ring canal so that water-vascular fluid may
be withdrawn from one part of the system to be supplied to another.
Saclike structures are associated with the
ring canal. These are polian vesicles, which act as reservoirs for
water-vascular fluid, and Tiedemann's bodies, which act as glands in which
wandering coelomocytes (coelom corpuscles) are formed.
Embryology
Most echinoderms have an indirect
development with a prolonged, food-gathering larval stage (Fig. 3). The larva
feeds, usually on microscopic phytoplankton, using a ciliary mechanism. Two
well-marked larval types occur: the pluteus group, with long-armed, bilaterally
symmetrical, easel-shaped forms, common to ophiuroids and echinoids; and the
auricularia group, barrel-shaped forms with a winding ciliated band which may be
produced into lobes. The latter group is common to asteroids and holothuroids.
In most asteroids the auricularia stage is followed by a similar but more
complex larva, the bipinnaria, or sometimes also by an anchored final larval
stage, the brachiolaria. Surviving crinoids have essentially a direct
development, sometimes with a simple yolky larva, a vitellaria, which does not
feed. This is also the case for several members of other extant echinoderm
groups. See also: Invertebrate
embryology
Phylogeny
The auricularia larva presents close and
striking resemblances to the tornaria larva of some enteropneusts, and the
enterocoelous development parallels that in primitive chordates. Hence
echinoderms and chordates have long been regarded as related. However, the
significance of similarities in the larvae of echinoderms and protochordates
must be viewed in the context of molecular, morphological, and paleontological
research. It seems likely that the similarities between the larvae of ophiuroids
and echinoids, and asteroids and holothuroids, are due to convergent evolution
and not to common evolutionary origins. The results of paleontology, molecular
studies, and morphology indicate that ophiuroids and asteroids are closely
related. Therefore it follows that within the phylum larval similarities do not
indicate phylogenetic affinities. It is inadvisable to try to extrapolate beyond
the phylum, so as to infer phylogenetic affinity between hemichordates and
echinoderms solely on the ground that the auricularia resembles the tornaria. E.
Marcus expressed the opinion that indirect development, with possession of
pelagic larvae, must be prototypical for echinoderms and protochordates, and the
asteroids and ophiuroids must be closely related, and therefore broad
phylogenetic conclusions cannot be drawn on the basis of their larvae. L. Hyman
grouped the extant echinoderms as their larval similarities suggest, and
concluded that the arrangement adopted by paleontologists must be wrong. H. B.
Fell and later authors have reaffirmed that the paleontological evidence
overrules nebulous embryological considerations.
David L. Pawson
Andrew C. Campbell
Ecology and
Feeding
Echinoderms occur everywhere in the world's
oceans, but they are usually rare in areas where the salinity of the water is
greatly reduced or where pollution levels are significant. In many areas,
echinoderms are dominant invertebrates in terms of numbers or biomass, and
locally sea stars can be the top predators. On a rocky shore, predatory sea
stars and vegetarian sea urchins may be common on and under rocks. Brittle stars
are typically concealed under rocks; they may emerge at night to forage for
small organisms. In deeper water, where conditions are suitable for suspension
feeding, huge aggregations of brittle stars or feather stars may occur, with
their arms extended into the water for feeding. Echinoderms flourish on and near
coral reefs, and it is in these tropical areas that they can achieve their
greatest diversity. In polar regions, especially around Antarctica, echinoderms,
along with sponges, dominate in most habitats (Fig. 4). Sandy to muddy bottoms
may be populated with large numbers of burrowing detritus-eating sea urchins,
long-armed brittle stars, and sea cucumbers. In such habitats, burrowing sea
cucumbers may extend their sticky tentacles into the surrounding water to
capture small drifting organisms. In deeper water, below 100 m (330 ft), stalked
sea lilies can be common on hard substrates, efficiently suspension-feeding with
their arms deployed in the form of a parabolic bowl. In the deep sea,
echinoderms can also be dominant; on abyssal (relating to great ocean depths)
plains, sediment-swallowing sea cucumbers can make up more than 95% of the total
biomass on the sediment surface. Most echinoderm groups occur in the deep sea to
depths in excess of 9000 m (5.6 mi).
David L. Pawson
Fig. 4 Echinoderms in the deep sea near
Antarctica at a depth of 595 m (1950 ft). Three sea cucumbers (Scotoplanes
globosa) are feeding on seafloor sediments. At bottom right is a brittle star,
probably Ophiomusium species. Scattered on the seafloor but barely visible are
several feather stars. (Courtesy of the U.S. National Science
Foundation)
Fossils
Echinodermata are an especially important
group of fossil invertebrates. Except for reworked fragments found in some
nonmarine deposits, echinoderm remains occur exclusively in strata laid down on
sea bottoms—chiefly those of shallow seas. Many of these deposits, ranging in
age from Cambrian to late Tertiary, contain abundant echinoderm fossils. In
addition, fossil echinoderms undoubtedly are widely distributed beneath all
oceans, although remains of these organisms in deep-water sediments, even of
recent origin, are virtually unknown because of their inaccessibility.
The echinoderms are well adapted to
preservation as fossils owing to their abundant calcareous skeletal elements.
Paleontological importance of the group is explained partly by this fitness but
more by the diversity of their kinds, the generally short-lived existence and
wide geographic distribution of most recognized taxonomic units, and the
clearness with which evolutionary trends can be defined.
Classification and
Description
For some years, fossil and living
echinoderms have been arranged into four or five subphyla. As formal groupings,
the subphyla have been largely abandoned, for it is recognized that the
subphylum definitions and distinctions are artificial rather than truly
reflecting phylogeny. However, the subphylum names are useful for grouping the
various echinoderm classes, and they are used here in an informal sense:
homalozoans, crinozoans, echinozoans, and asterozoans. Similarly, the terms
eleutherozoan (free-moving) and pelmatozoan (sedentary or sessile), previously
used in formal classifications, are useful today in categorizing the lifestyles
of echinoderms. See also:
Eleutherozoa; Pelmatozoa
Homalozoans
The distinctive features that separate the
extinct homalozoans (or carpoids) from all other Echinodermata are a complete
lack of radial symmetry in arrangement of skeletal parts and the flattened body
form, from which one or more specialized slender appendages may extend. Plates
enclosing the body are irregular in shape and size and vary in number between
forms. There are four quite dissimilar classes of carpoids: Homostelea,
Ctenocystoidea, Stylophora, and Homoiostela. Some experts, especially R.
Jefferies, believe that some of the homalozoans, the Stylophora in particular,
are not echinoderms at all, but are “calcichordates,” groups forming the stem of
the chordates. There has been considerable debate in the scientific literature
about this matter, with persuasive evidence presented on both sides. Most
paleontologists tend toward the idea that all homalozoans are true echinoderms.
The homalozoans range in age from earliest Cambrian to Carboniferous. See also: Carpoids; Homalozoa
Crinozoans
The crinozoans evidently include types of
echinoderms least modified from the ancient progenitors of the phylum.
Crinozoans range from Cambrian to Recent, but only the Crinoidea, abundant in
Paleozoic formations, have survived to the present. In some classifications, the
Crinoidea and Paracrinoidea are treated as the subphylum Crinozoa in the strict
sense, and the other classes are grouped together in the subphylum Blastozoa. A
characteristic crinozoan feature is orientation of the body with the oral
(ventral) side directed upward and aboral (dorsal) side downward; thus, although
commonly used, the designations ventral and dorsal are not suited to these
particular echinoderms (Figs. 5c and 6). There are nine classes of crinozoans:
(1) Eocrinoidea (the crinozoan stem group)
and (2) Paracrinoidea are crinozoan groups attached by a stalk. They have ovoid
bodies enclosed by irregularly arranged plates that commonly lack well-developed
radial symmetry and possess a combination of characters typical of the two
following classes. (3) Diploporita and (4) Rhombifera (previously grouped in the
polyphyletic Cystoidea, with saclike bodies, and each possibly polyphyletic) are
a diverse group of primitive crinozoans distinguished especially by pores or
tubular canals that penetrate irregularly arranged plates enclosing the body.
Attachment to the substrate was by means of a stem composed of superposed
discoid plates. (5) Crinoidea (sea lilies) include the most abundant crinozoans,
most of the fossil forms anchored to the sea bottom by means of a short to very
long stem and distinguished by upwardly directed arms which functioned for
gathering food. Some ancient and most modern crinoids are stemless, known as
feather stars, and adapted for a free-living existence. True radial symmetry and
regularity of skeletal construction are attributes that distinguish crinoids.
(6) Blastoidea (budlike forms) are small to medium-sized crinozoans having a
highly developed radial symmetry and very regular arrangement of the few plates
surrounding the body. The body was attached to the substrate by a very slender
stem, which had a circular cross section. (7) Coronoidea is a small group of
middle Paleozoic crinozoans, similar to blastoids and probably ancestral to
them. Coronoids differ in the structure of the ambulacral plate and possess
erect, as opposed to recumbent, ambulacra. (8) Parablastoidea is another small,
principally Ordovician class of crinozoans with cystoid-like thecae but with
uniserial ambulacra. (9) Edrioblastoidea comprises a single Ordovician genus
with a theca that is blastoid-like in shape and symmetry but lacks brachioles or
hydrospires (or pore rhombs). See
also: Crinozoa
Fig. 5 Representative types of echinoderms. (a)
Regular echinoid (Lytechinus, Recent), oblique aboral view, right half with
spines removed, showing two ambulacra and three interambulacra. (b) Diagrammatic
section through a, showing the stoutly built test. (c) Crinoid showing the stem
and three arms, calyx sectioned. (d) Asteroid (Dermasterias, Recent), aboral
oblique view. (e) Diagrammatic section through the ray at left in d and opposite
the interambulacrum. (f) Holothuroid, oral view showing tentacles around the
mouth. (g) Holothurian, lateral view.
Fig. 6 Fossil pelmatozoan echinoderms. (a)
Rhombiferan cystoid (Echinoencrinus, Ordovician). (b) Diagrammatic oblique view
and section through a cystoid pore rhomb (half of one rhomb toward front
bisected by a suture between plates). (c) Oblique view and section of a
diploporitan cystoid plate. (d) Section through the ambulacrum of a blastoid
(Pentremites, Mississippian) showing hydrospires beneath the lancet and side
plates. (e) Blastoid (Pentremites) with brachioles restored along the side of
one ambulacrum. (f) Edrioasteroid (Carneyella, Ordovician), oral view. (g)
Section through a stalkless edrioasteroid. (h, i) Flexible crinoid (Taxocrinus,
Mississippian), posterior side and partial plate diagram showing the right
posterior plane of bilateral symmetry defined by the infrabasal circlet. (j–p)
Inadunate crinoids, some showing anal sacs and parts of arms: (j) Carabocrinus,
Ordovician; (k) Botryocrinus, Devonian; (l) Cyathocrinites, Mississippian; (m)
Cupulocrinus, Ordivician; (n–p) Delocrinus, Pennsylvanian. (q) Diplobathrid
camerate crinoid (Ptychocrinus, Ordovician), plate diagram. (r, s) Monobathrid
camerates: (r) Macrostylocrinus, Devonian; (s) Periechocrinites, Mississippian,
plate diagrams showing noteworthy distinctions in basal and radial circlets.
(Parts f, g after L. H. Hyman, The Invertebrates, vol. 4: Echinodermata,
McGraw-Hill, 1955)
Eocrinoids
This small group of Cambrian to Silurian
crinozoans combines characteristics of cystoids and crinoids, yet differs
significantly from both. They resemble cystoids in their mode of branching of
the ambulacral grooves and ventrolateral location of the anus but lack thecal
pores or distinct pore rhombs; they are like crinoids in plate structure and
similarity of plate arrangement in the calyx (plated body). The eocrinoids have
stems and unbranched or bifurcating arms. They were probably ancestral to all
other crinozoans. See also:
Eocrinoidea
Paracrinoids
Paracrinoidea are stem-bearing echinoderms
that also combine features of cystoids and crinoids, but in a manner quite
unlike that of the eocrinoids. The paracrinoids, now known only from Middle
Ordovician to Lower Silurian deposits, have numerous plates of the calyx that
are not arranged in series and that lack a ventrally differentiated area
corresponding to the tegmen of crinoids. The plates have a cystoid-like pore
structure, but the arms are comparable to those of crinoids.
Cystoids
The Ordovician to Devonian cystoids,
comprising the classes Diploporita and Rhombifera, are extinct crinozoans of
globose, subcylindrical, or flattened ellipsoidal form that are characterized
mostly by irregularity of the body plates (Fig. 6a) and a short, weak stem. A
variable number of slender appendages (brachioles) on the upper side of the
calyx, supplemented by ambulacral grooves, served for gathering food. The mouth
was located centrally at the summit of the calyx, and an anus was fairly well
down on one of the sides, which accordingly is defined as posterior. Other small
orifices, interpreted as hydropore and gonopore, occur near the anus. In one
class, Diploporita, the numerous calyx plates are perforated by numerous pairs
of minute tubular openings that allow seawater to circulate through them (Fig.
6c). Remaining cystoids are named Rhombifera (rhomb-bearing forms) because some
or all of their relatively few calyx plates are penetrated by rhomb-shaped
groups of tubes running parallel to each other and to the plate surfaces (Fig.
6a). The two halves of any rhomb lie on adjoining plates, with the tubes
crossing the plate boundaries at right angles (Fig. 6b). The pattern of calyx
plates suggests that the blastoids and crinoids may be descendants of
rhombiferan cystoids. See also:
Rhombifera
Crinoids
In terms of abundance of fossil remains, the
crinoids outrank all other echinoderms combined (Fig. 5c). There are about 5000
extinct species and at least 700 living kinds. Some Paleozoic deposits hundreds
of feet thick in areas measured in hundreds of square miles are largely composed
of fossil remains of trillions of crinoids. See also: Crinoidea
Skeletal
features
Although many modern and some ancient
crinoids are stemless as adults, this group of pelmatozoans typically is
attached to the sea bottom by a more or less elongate stem composed of
superposed calcareous discs (columnals) perforated centrally by a circular,
pentagonal, or pentastellate canal. At the opposite extremity of the stem, which
exceptionally may be 15 m (50 ft) tall, is the crinoid body, encased in
regularly arranged plates and surmounted by branched or unbranched free-moving
arms. The conjoined plates below the free arms make up the so-called dorsal cup;
this cup, along with the plates of the ventral surface composing the tegmen,
makes up the crinoid calyx (Fig. 6h–s). The calyx and its attached arms are
termed the crown. The mouth is located on the ventral surface. The anus lies on
the tegmen, is raised above it on an anal sac (Fig. 6k–m), or may lie on the
side of the dorsal cup. The posterior side of the crinoid is defined by the
position of the anus in one of the interrays or by extra plates introduced in
such a position on one side of the dorsal cup; the ray opposite to the posterior
interray is defined as anterior. This establishes a plane of bilateral symmetry
that more of less modifies the fundamental radial symmetry of the crinoid (Fig.
7a).
Fig. 7 Bilateral symmetry developed in various
echinoderms. Anterior and posterior directions are indicated where
distinguished. All diagrams represent aboral views. (a) Crinozoans (in general,
most crinoids, cystoids, edrioasteroids); letters denote ray designations
according to the Carpenter (P. H. Carpenter, 1884) system: A, anterior; B, right
anterior; C, right posterior; D, left posterior; E, left anterior. (b)
Blastoids, heterocrinoids, with subordinate bilateral symmetry in the left
posterior plane. (c) Homocrinoids, with primary bilateral symmetry in the left
anterior plane. (d) Glyptocrinoids, flexible crinoids, and rhombiferan cystoids,
with subordinate bilateral symmetry in the right posterior plane. (e)
Holothuroids. (f) Regular echinoids, rays marked according to the Lovén (S.
Lovén, 1874) system, III being considered anterior. (g) Irregular echinoids,
with prominent bilateral symmetry in the left posterior plane of crinozoans. (h)
Asteroid, with subordinate bilateral symmetry in the left anterior
plane.
The plates of the crinoid dorsal cup are
arranged in a regular pattern of successive circlets, but with differences in
various groups that furnish a basis for classification. Each circlet normally
contains five plates. At the base of each ray is a radial plate. Beneath the
circlet of radials are five basals that are interradial in position; some
crinoids have a still lower circlet of infrabasals that alternate with the
basals and hence occur in radial position. Crinoids with a single circlet of
plates below the radials are termed monocyclic (Fig. 6r and Fig. 6s), and those
with two circlets are dicyclic (Fig. 6h–q).
In some crinoids the tegmen is stoutly
constructed of small, irregularly arranged plates, while in others it consists
of a flexible, leathery integument that may be studded with calcareous ossicles.
Five subequal oral plates larger than others of the tegmen may occur
interradially around the mouth. The arms of crinoids are extremely variable in
plan and construction. They may be unbranched or moderately to highly branched,
with or without very numerous tiny branchlets called pinnules, and composed of a
single or double series of arm plates (brachials). These characters, along with
the mode of articulation among plates of the rays, are important also in
classification.
Main
types
Four subclasses of crinoids are recognized,
three of them distributed from Ordovician to Permian; one of the three
(Inadunata) persists to Middle Triassic. The fourth subclass (Articulata) ranges
from the Mesozoic to present day.
The Inadunata (“not united” forms, referring
to lack of incorporation of lower arm plates in dorsal cup) are crinoids with a
relatively small dorsal cup containing one or two circlets of plates below the
radials and having the arms entirely free above the cup (Fig. 6j–p). They
include 1750 or more species that exhibit great variety in form and evolutionary
trends. In a majority the anteroposterior plane of bilateral symmetry is well
marked, without other deviation from a regular pentameral plan (Fig. 7a), but in
one group (superfamily Homocrinacea) a surprising degree of bilateral symmetry
was developed in the plane of the left anterior ray (Fig. 7c). Some of these
crinoids have a strongly downbent crown that was hinged on the summit of the
stem. In another group (superfamily Heterocrinacea) a subordinate plane of
bilateral symmetry is oriented in the left posterior plane (Fig. 7b). See also: Inadunata
The Camerata (chamber or “box” forms) are
most numerous among ancient crinoids in both variety and quantity of individuals
(Fig. 6q–s). More than 2500 species of these fossils have been described,
two-thirds of which come from Mississippian rocks alone. The camerates are
distinguished by the stout construction of their calyx, which incorporates lower
ray plates and interradials in the dorsal cup, and subtegminal location of the
mouth. Types with both one and two circlets of plates beneath the radials are
common. Anteroposterior bilateral symmetry is developed almost universally in
these crinoids as a modification of the dominant pentameral pattern (Fig. 7a); a
secondary plane of bilateral symmetry directed through the right posterior ray
prevails in the monobathrid suborder Glyptocrinina, as in all flexible crinoids
(Fig. 7d). See also: Camerata
The Flexibilia (flexibles) are a distinctive
assemblage of exclusively dicyclic crinoids characterized by movable ligamentous
union between most of the plates and several constant features in the
organization of the calyx (Fig. 6h and Fig. 6i). They include approximately 300
described species, all of which exhibit a secondary bilateral symmetry in the
right posterior plane in addition to their generally well-marked primary
bilateral symmetry directed anteroposteriorly (Fig. 7d). See also: Flexibilia
The Articulata (forms divided into joints)
comprise Mesozoic and Cenozoic crinoids, represented by about 500 fossil and
about 700 living species. The stalked articulate crinoids have been classified
into four orders: Millericrinida, Cyrtocrinida, Bourgeticrinida, and Isocrinida.
All of these orders have fossil and extant representatives. Some experts have
abandoned the order categories for the 100 species of living stalked crinoids
and prefer to arrange them into 11 families. There are three orders of unstalked
crinoids: Uintacrinida and Roveacrinida are extinct, while the Comatulida
(feather stars) has about 600 living species. Extant sea lilies are confined to
depths below about 100 m (330 ft). The stalk is usually attached to a hard
substratum—a rock, or a piece of shell embedded in soft mud—by means of a
terminal disc cemented to the substratum or by means of claw-like whorls of
cirri. In a few species, the end of the stalk is branched and rootlike for
anchoring the stalk in soft sediments. When detached from the substratum and
lying on the seafloor, sea lilies are capable of slowly moving to a new site. In
contrast, the feather stars are highly mobile, often capable of active swimming
by thrashing of the arms in an up-and-down motion. Feather stars range from
shallow waters to great ocean depths. They are common on coral reefs, and in
certain suitable habitats can reach population densities of hundreds of
individuals per square meter. Sea lilies and feather stars are passive
suspension feeders; they extend their arms into the water and capture small
drifting organisms and particulate matter on their sticky tube feet. Captured
items are conveyed to the mouth along food grooves by means of ciliary action.
In extant crinoids, the gonads develop inside the pinnules near the bases of the
arms, causing conspicuous swelling. At the appropriate time for breeding, the
pinnules burst to release eggs or sperm into the surrounding seawater. See also: Articulata (Echinodermata)
Blastoids
Blastoidea, Ordovician to Permian, display a
regular, fivefold radial symmetry (Fig. 6e), on which are superposed a first
order of bilateral symmetry in the anteroposterior plane and a second order
directed through the left posterior ray (Fig. 7b). The calyx of average-size
specimens is small, with diameter of about 15 mm (0.6 in.) and height of 20 mm
(0.8 in.). It is composed of 23 or 24 plates, of which five (lancet plates) are
not visible externally on unweathered specimens. Within forks of the radials are
a multitude of very diminutive so-called side plates that conceal the lancets;
they form ambulacral areas bordered laterally by rows of threadlike free armlets
(brachioles) that function as food gatherers. Particles of food are conducted to
midlines of the ambulacra and thence upward to the mouth, which is at the center
of the ventral surface. Beneath the ambulacra are extremely delicate,
longitudinally folded, calcareous lamellae that enclose narrow troughs for
circulation of water admitted through pores at the base of the brachioles; these
structures, termed hydrospires, correspond to the slitlike parts of pore rhombs
in the rhombiferan cystoids (Fig. 6d). More than 500 species have been
described. See also: Blastoidea
Asterozoans
The asterozoans are star-shaped, free-moving
echinoderms with well-marked radial symmetry in arrangement of their skeletal
parts. They are easily recognized by their strongly developed arms and central
disk (Fig. 5d and Fig. 5e). The arms of most asterozoans, with their tube feet
functioning for locomotion and predation, are highly mobile. They range in age
from Early Ordovician to Recent but are more abundant and varied in modern seas
than in the fossil record. They are divided into four classes: Somasteroidea,
Asteroidea, Concentricycloidea, and Ophiuroidea. The extinct Somasteroidea
ranged from the Ordovician to the Devonian. Somasteroids resemble true sea stars
in general shape, with broad arms, but the arms are unique in having a pinnate
arrangement of rodlike plates. Asteroidea (sea stars) are distinguished by
prominent flexible rays that join the central disk without clearly shown
demarcation (Fig. 5d and Fig. 5e). Concentricycloidea are flattened and
discoidal with no evident arms. The Ophiuroidea (brittle stars) have long,
slender, snakelike arms which are sharply set off from the central discoid body.
They are very active echinoderms, able to crawl rapidly in any direction.
Somasteroids
These rare, broad-armed sea stars are
especially characterized by the featherlike arrangement of parallel, rodlike
plates in their wide petaloid rays. The rods extend outward from the medially
placed ambulacral plates. A food-carrying groove is contained in each ray. The
aboral side of the skeleton has a rough, coarse meshwork.
Asteroids
The sea stars have a long fossil history,
ranging back to the Ordovician. Sea stars are comparable to the ophiuroids in
their general stellate form (Fig. 5d and Fig. 5e, Fig. 7h). They differ from the
brittle stars in two main respects: lack of strongly marked separation between
the body and its radially disposed hollow arms, and the open ambulacral grooves
along the ventral side of the arms. The skeletal elements tend to be loosely
joined; consequently, asteroids are not well suited for fossilization in a
manner showing the skeletal arrangement of the entire animal. Sea stars may
range in size from a few millimeters to over a meter (3.3 ft) in diameter.
Ossicles along the ambulacra occur in two or four series differentiated as
ambulacrals and adambulacrals. Some asteroids lack obvious arms and are
essentially pentagonal with ambulacral grooves on their ventral (oral) side.
Others, such as the Heliasteridae, develop as many as 44 arms as they grow,
losing their fundamental pentameral symmetry but structurally resembling common
types of asteroids. A single madreporite lies on the upper (aboral) surface,
placed between the arms (interradially). An anus is present in the center of the
aboral surface in most groups and absent in others (notably the Paxillosida).
Behavior
Asteroids are universal symbols of the
ocean. They are conspicuous in a great variety of habitats, from mud to sand to
rock to coral reefs. In some areas they can be present in great numbers, causing
local habitat damage. The multiarmed, crown-of-thorns sea star, reaching 1 m
(3.3 ft) in diameter, is common on Indo-Pacific coral reefs. It can move across
reefs in large swarms, eating the soft coral tissues and causing short- to
long-term devastation of reefs. Some groups of sea stars, notably the
Paxillosida, have pointed rather than suckered tube feet, and they tend to
swallow prey whole (such as small clams and sand dollars), ejecting the shells
from the mouth when digestion is complete. The Forcipulatida are notorious
predators; a forcipulatid may attach its suckered tube feet to the two shells of
a clam or a mussel and then, for several hours if necessary, gently pull on the
two shells, until the bivalve's muscles tire, the shells open a fraction of an
inch, and the sea star drops its stomach into the bivalve, digesting the animal
inside its own shell. Some deep-sea asteroids are mud-swallowers, digesting from
the mud whatever organic material may be present.
Classification
The classification of the Asteroidea has
been revised by many authors over the past 50 years or so. In recent years, the
most commonly used classification of both fossil and extant forms is that
proposed by D. B. Blake. Extinct Paleozoic orders are Platyasterida,
Pustulosida, Hemizonida, and Uractinida. Post-Paleozoic sea stars include the
extinct order Trichasteropsida, and seven extant orders, all of which also have
fossil representatives: Paxillosida, Notomyotida, Velatida, Valvatida,
Spinulosida, Forcipulatida, and Brisingida. See also: Asteroidea; Notomyotida;
Paxillosida
Concentricycloids
The sea daisies comprise three extant
deep-sea species, two known from the Pacific Ocean and one from the Atlantic.
Sea daisies are flattened, discoidal echinoderms up to 15 mm (0.6 in.) in
diameter. The upper surface is covered with delicate overlapping plates; the
lower surface has a mouth frame and a single peripheral ring of tube feet. These
animals have been found only in association with pieces of wood on the deep-sea
floor. Presumably, they attach to the wood substratum using their suckered tube
feet. This class was discovered and diagnosed in 1986, and its formal status has
been debated ever since. Several scientists have argued persuasively that the
concentricycloids should be placed in the Asteroidea, while others maintain that
they represent a separate class of echinoderms.
Ophiuroids
The brittle stars or serpent stars composing
the Ophiuroidea are highly mobile echinoderms closely related to the sea stars.
They are distinguished readily by their external form, since the small, rounded
to pentagonal or scalloped disk is sharply set off from the symmetrically placed
long, slender arms that extend radially outward from it. The arms may be smooth
or spiny. Almost invariably they are five in number and unbranched, but in a few
kinds, such as the Recent basket stars (Gorgonocephalidae), the arms are
repeatedly bifurcated. All known fossil ophiuroids have simple arms. The
skeleton of the central disk is composed of many regularly arranged plates,
without any deviation on either the aboral or oral surfaces from perfect
pentamerous symmetry. The mouth is at the center of the oral surface; an anus is
lacking. A single interradial madreporite is present on a plate near the mouth;
in some basket stars there are five interradial madreporites around the outer
margin of the mouth skeleton. The arms are solid, not hollow as in asteroids.
The arm skeleton is distinctive in being internal, consisting of “vertebrae”
formed from fused ambulacral ossicles. Brittle stars are common in a great
variety of habitats—living under rocks, concealed in coral reefs, or scattered
in enormous numbers across great expanses of mud on the deep-sea floor. Many
brittle stars are selective detritus feeders; others feed by extending their
arms into the seawater and capturing small drifting organisms or particles on
their sticky tube feet. Recent investigations have shown that certain brittle
stars have well-developed eyes in their upper-arm skeletons, and they can
respond rapidly to changing light regimes, seeking shelter in crevices as
necessary.
Fossil ophiuroids are known from Lower
Ordovician to Pleistocene (Recent), but they are not abundant. Fewer than 100
species have been described from Paleozoic, Mesozoic, and pre-Recent Cenozoic
formations, as compared with approximately 2000 known living species. Ophiuroids
are arranged in four orders: the extinct Stenurida (Ordovician to Devonian) and
Oegophiurida (Ordovician to Carboniferous), and the extant Phrynophiurida and
Ophiurida (Ordovician to Recent).
See also: Ophiuroidea
Echinozoans
These echinoderms generally have an ovoid or
globose body, and they lack armlike appendages. Some echinozoans may have a low
discoid body or an elongate cylindrical to almost wormlike form. All have
conspicuous radial symmetry of their skeletal parts, and in almost all the
symmetry is pentamerous. The skeleton of some echinozoans consists of a rigidly
constructed test to which movable jaw parts and external spines are attached,
whereas in others all skeletal parts either are joined together flexibly or are
reduced in size and separated from one another by leathery tissue. The
echinozoans comprise seven classes: Helicoplacoidea, Camptostromatoidea,
Edrioasteroidea, Cyclocystoidea, Ophiocistioidea, Echinoidea, and Holothuroidea,
the first five of which are long-extinct groups, while the others are
represented by both fossil and living forms. (1) The Helicoplacoidea are Lower
Cambrian forms with a flexible, expansible test composed of spirally arranged
plates. (2) The Lower Cambrian Camptostromatoidea are conical or domal animals
with plates of varying size overlapping on the lower theca. (3) Edrioasteroidea
(“sessile sea stars”) range from the Ordovician to the Permian. They have a
many-plated test, ranging from rigid to somewhat flexible, of generally discoid
shape with five distinct, straight or curved rays on the upper surface. Some
were able to move about, but nearly all are found attached to some hard foreign
object, such as a brachiopod or other invertebrate shell. (4) The Cyclocystoidea
are Ordovician-to-Devonian echinoderms with a disk-shaped body, the upper
surface of which is covered by plates arranged in concentric rings. (5) The
Ophiocistioidea are Ordovician-to-Devonian echinoderms that differ from other
classes in having large fine-plated tube feet, but share some skeletal features
with the Holothuroidea, with which they have sometimes been aligned. (6) The
Echinoidea (sea urchins) are characterized by a rigid to flexible skeleton of
varied shapes, though mostly ovoid to subglobose, covered by numerous long or
short movable spines (Fig. 5a and Fig. 5b). Pentameral symmetry is strongly
developed, modified in all orders by more or less distinct bilateral symmetry
and in some by differentiation of anterior and posterior regions. Very abundant
in present-day seas, sea urchins range from Early Ordovician onward. (7) The
Holothuroidea (sea cucumbers) are soft-bodied, leathery-skinned echinozoans with
an elongate cylindrical body and skeletal parts usually consisting of discrete,
generally microscopic, ossicles (“little bones”) embedded in the body wall and
internal tissues (Fig. 5f and Fig. 5g).
See also: Echinozoa
Edrioasteroids
A unique assemblage of mainly early
Paleozoic attached echinoderms (Fig. 6f and Fig. 6g), Edrioasteroidea were
distributed from Lower Cambrian to Upper Carboniferous. They had no stem for
anchorage but were fixed by their entire base. The upper (ventral) side was
covered by many flexibly joined plates, which are sharply differentiated into
rows of paired ambulacrals with adjoining adambulacrals and irregular
interambulacrals. The five ambulacral tracts curve outward from the centrally
located mouth, with their extremities often deflected rather strongly in a
consistent way. The anus lies in one of the interrays that accordingly is
regarded as posterior.
Helicoplacoids
Numerous representatives of Helicoplacoidea
have been found in the oldest fossil-bearing strata (Olenellus Zone) of the
Lower Cambrian at localities in western Nevada and southern California. The body
is ovoid to pyriform, enclosed in a test of loosely joined, spirally arranged
rows of elongate plates, with columns, interpreted as ambulacral, and
intervening ones, as interambulacral, originating at an apical pole and
extending to the opposite oral pole. The test could be distended and retracted
so as to change its shape and volume. Other echinoderms associated with these
oldest echinozoans include representatives of the pelmatozoan class
Eocrinoidea. See also:
Helicoplacoidea
Echinoids
The class Echinoidea is the most important
group of fossil eleutherozoan echinoderms (Fig. 5a and b; Fig. 8). They are
distinguished from crinozoans by freedom from a sessile mode of life, and they
share with the holothuroids a downward orientation of their oral surface. Modern
species of echinoids total approximately 800, as shown by T. Mortensen's
several-volume monograph (1928–1952) covering all known living kinds. In
comparison, 2000 species of sea stars, 2000 species of brittle stars, and 1200
species of sea cucumbers are alive today. All eleutherozoan groups are known as
far back as Ordovician time, and yet fossil echinoids are more numerous than all
others of the assemblage combined. Such disparity reflects inequality in fitness
of the different sorts of skeletons to be preserved. Not all parts of echinoid
skeletons may be found intact. Multitudes of movable spines attached to the test
during life tend to be separated and scattered, as does the echinoid masticatory
apparatus, which is known as the Aristotle's lantern. Thus, knowledge of the
entire skeletal structure of most fossil echinoids is incomplete. See also: Echinoidea
Fig. 8 Fossil echinoids. (a) Regular form
(Echinocrinus, Mississippian), aboral view showing the anus and periproct, wide
interambulacra and narrow ambulacra. (b) Irregular form (Eupatagus, Paleogene),
aboral view, left side showing an unweathered appearance with many spine bases,
right side showing the plate structure with ornamentation omitted; anterior ray
directed upward. (c, d) Types of echinoid spines: (c) Porocidaris, Paleogene;
(d) Balanocidaris, Jurassic.
Morphology
The echinoid test is typically globular,
unevenly ovoid, or discoid in shape (Figs. 5 and 8). Mainly it is formed by 10
meridionally disposed bands of plates, of which five are ambulacral,
distinguished by porelike openings between or through the plates for passage of
the tube feet; the five bands of plates that alternate with the ambulacrals are
interambulacral and lack perforations for tube feet. In most echinoids, both
living and fossil, the plates are joined together rigidly, but in a minority the
union is somewhat flexible and imbricating (having overlapping edges). Virtually
all post-Paleozoic echinoids have a constant arrangement of the ambulacral and
interambulacral groups of plates, each of the 10 bands comprising a double
column, making 20 columns of plates in the test as a whole. The plates are
widest at the equator (ambitus) of the test and increasingly narrow toward the
oral and aboral poles.
The mouth, located on the underside, is
surrounded by a peristome, an area of naked integument or flexibly united small
plates. The anus generally is placed on the aboral side of the test opposite the
mouth, but it may be shifted backward to a posterior position or even to the
oral surface. The anus is surrounded by a bare or small-plated periproct.
Another part of the test important for orientation and classification is the
so-called apical system of 10 special plates at the aboral pole, five so-called
ocular plates alternating with five genital plates. One of these genital plates
is an interambulacrally placed sieve plate (madreporite) that is a landmark for
identifying homologous parts, not only among diverse kinds of fossil and living
echinoids but among echinoderms generally. Fossil remains of echinoids include
numerous spines not associated with the test that bore them. Even so, the spines
are commonly distinctive and useful as paleontological tools (Fig. 8c and Fig.
8d).
Orientation
Seemingly, there should be little reason for
discussing the orientation of echinoids because the downward-directed oral
surface clearly is ventral and, at least among irregular echinoids which are
characterized by obvious bilateral symmetry, locomotion consistently is in a
single direction along the path defined by the plane of this symmetry. If such
direction of movement is considered to be forward, the ambulacral ray on this
side of the test must be anterior and the opposite interambulacrum is posterior
(Fig. 7g). In irregular echinoids, such as the spatangoids, the spines nearly
all point backward. In many irregular genera, the mouth is found to have shifted
forward and the anus rearward. These observations have led to adoption by
specialists of a scheme of numerical designation of the ambulacral and
interambulacral rays, introduced by S. L. Lovén (1874) and known as the Lovén
system (Fig. 7f). It is applied to fossils as well as living echinoids and to
both irregular and regular types. Orientation of the regular echinoids seemingly
should not be easy, since they move in any direction with equal ease and without
detectable preference; also, except for the madreporite, the test has perfect
radial symmetry. Inclusion of the regulars is possible only by use of the
off-center location of the madreporite on the aboral surface as a reference
point, assuming that relative position is the same in all echinoids. The
interambulacrum with the madreporite is identified as number 2 of the Lovén
system.
Among crinozoans, which almost certainly
embrace the ancestors of echinoids, the posterior interradius is readily and
positively identified. If this orientation is applied to echinoids, a
discrepancy becomes evident at once, for the adopted anteroposterior plane of
echinoids clearly is that coinciding with the left posterior ray of the
crinozoans (Fig. 7e and Fig. 7f). Interambulacrum 2 (Lovén), considered as right
anterior by echinoid students, is equivalent to the posterior interambulacrum of
crinoids, for example. This does not mean that echinoid orientation is wrongly
conceived but merely that evolution of these echinoderms has pursued a divergent
path of its own, which incidentally duplicates the subordinate plane of
bilateral symmetry in some crinozoans (blastoids, heterocrinoids).
Habitats and
behavior
Echinoids can be extremely common from the
rocky intertidal down to abyssal ocean depths. The irregular urchins, such as
sand dollars, sea biscuits, and heart urchins, are always associated with soft
substrates—sand, sandy mud, or mud. Sand dollars and sea biscuits live at the
surface of sand or burrow just beneath the surface. Many of these echinoids
ingest sand, obtaining sustenance from small organisms, such as bacteria,
associated with the sand grains. Heart urchins burrow into the substratum and
remain buried, feeding on particulate matter in the mud. The regular urchins are
usually associated with hard substrates, typically rock, coral, or rubble, but
numerous species may occur on sand or mud in quiet waters. Most of the regular
urchins are vegetarians, feeding primarily on algae. Typically, echinoids
reproduce by liberating eggs or sperm into the seawater from genital pores on
top of the body. There are usually five pores, but in irregular urchins there
may be four, three, or two pores. Fertilization of eggs takes place in the
seawater, and the developing embryo becomes a planktonic larva. Within a few
weeks, the urchin develops on the left side of the larva, breaks away, and sinks
to the bottom as a juvenile. Some urchins have a nonfeeding yolky larva, and in
others the larval stage is omitted altogether, the young being brooded by the
female parent in special chambers.
Classification
Former taxonomic arrangement of the
echinoids recognized two main subclasses, the regular echinoids (Regularia) and
irregulars (Irregularia). Present knowledge, based on fossils as well as living
forms, shows that this arrangement is artificial. The so-called regulars are a
composite assemblage that on the one hand contains several extinct primitive
kinds, exclusively Paleozoic, and on the other, advanced forms of modern type,
all Mesozoic and Cenozoic, that include the stocks (Diadematacea) from which
irregular echinoids were derived. Accordingly, classification was considerably
revised (J. W. Durham and R. V. Melville, 1957; A. B. Smith, 1984; and others)
so as to take account both of Mortensen's comprehensive work on living echinoids
and of evidence from paleontology.
Ophiocistioids
Ophiocistioidea is an aberrant echinozoan
group containing five known genera distributed from Lower Ordovician to Middle
Devonian. The chief peculiarity of the group is the presence of numerous armlike
appendages attached to the oral surface in an ambulacral position. They are
interpreted as much-enlarged podia that are covered by abundant minute plates, a
character seemingly shared with at least one early holothurian. Small ossicles
in the ophiocistioid body wall closely resemble ossicles of holothurians. An
early relationship with the holothuroids has been suggested.
Holothuroids
The sea cucumbers, or Holothuroidea, are a
group of echinozoans that must have diverged very early from other echinoderm
stocks. They clearly exhibit a fivefold radial symmetry characteristic of the
phylum but otherwise differ radically from any other echinoderm assemblage. They
are greatly elongated along the oral-aboral axis so as to have a subcylindrical
form, and they lie on one of their sides identified as ventral because it is in
contact with the substratum (Fig. 5f and Fig. 5g, Fig. 7e). The mouth,
surrounded by feeding tentacles, lies at one extremity and the anus at the
opposite end. The tentacles can assume a variety of forms, from richly branching
(dendritic) to shield-shaped, to featherlike, to digitiform (fingerlike). Sea
cucumbers are unique among living echinoderms in possessing an internal
madreporite and a single gonad. In a small percentage of species, the body is
enclosed in a test of conspicuous plates, but in most the body is flexible, the
skeleton consisting of microscopic calcareous plates loosely distributed in the
dermis. When the animal dies, decay of the uncalcified tissues liberates the
skeletal parts, which almost invariably become scattered about. Less than a
dozen whole-animal fossil holothuroids have been described. The fossil remains
of the holothuroids usually consist only of discrete microscopic ossicles. Even
so, about 200 species of these ossicles have been described from rocks ranging
from Devonian to Pleistocene. Most of the fossil forms can be referred to one or
the other of the orders of extant holothuroids. Sea cucumbers range from rocky
shores to the greatest ocean depths. On the deep-sea floor they can be present
in enormous numbers and may comprise 95% of the total weight of animals on the
seafloor. Typically sea cucumbers ingest the soft sandy to muddy substrata on
which they occur. Such feeding may be nonselective mud-swallowing or selective
detritus ingestion. The sea cucumbers with branching tentacles are suspension
feeders, extending their sticky tentacles into the water and capturing small
drifting organisms or organic particles.
About 20 tropical species are prized as
food, especially in Asia, and as a consequence these species are more or less
fished out throughout the tropics. Six orders of living sea cucumbers are
recognized, based upon characters of the feeding tentacles and skeleton. See also: Holothuroidea
Origin and
Phylogeny
The evolution and relationships of the major
groups of echinoderms are under active discussion, and several major issues have
yet to be resolved. The extraxial/axial theory (EAT) of B. David and R. Mooi
offers a new approach to discussion of skeletal homologies. Echinoderms evolved
very rapidly near the beginning of the Paleozoic Era, and Lower Cambrian
deposits contain such divergent branches of the phylum as homalozoans
Helicoplacoidea, Edrioasteroidea, and Eocrinoidea (Fig. 9). These are primitive
types of echinoderms. Cystoids, crinoids, and blastoids, as well as all
recognized main groups of asterozoans and echinozoans (except holothurians),
appear in Ordovician strata. During the Paleozoic, numerous well-marked
evolutionary trends are discernible in nearly all echinoderm groups, including
free-moving forms (especially echinoids) as well as crinozoans. Many small
classes of echinoderms became extinct during the Paleozoic, and the surviving
groups, especially the crinoids, lost many members at the Late Permian mass
extinction. All groups of modern echinoderms have their origin in early
Paleozoic stocks, and the lines of their phylogeny are mostly indicated by the
fossil record. Echinoids predominate in Mesozoic and Cenozoic echinoderms.
David L. Pawson
Raymond C. Moore
J. John Sepkoski, Jr.
Fig. 9 Phylogeny and diversity range chart for
the Paleozoic echinoderms. (After C. R. C. Paul and A. B. Smith, The early
radiation and phylogeny of echinoderms, Biol. Rev., 59:443–481,
1984)
Bibliography
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B. David and R. Mooi, Embryology supports a new theory of skeletal homologies for the phylum Echinodermata, C.R. Acad. Sci. Paris Ser. 3, 319:577–584, 1996
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L. H. Hyman, The Invertebrates: Echinodermata, vol. 4, 1955
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D. T. J. Littlewood, Echinoderm class relationships revisited, in Echinoderm Research, ed. by H. Roland et al., A. A. Balkema, Rotterdam, 1995
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C. G. Messing, Living comatulids, Paleontol. Soc. Pap., 3:3–30, 1997
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R. Mooi, Not all written in stone: Interdisciplinary syntheses in echinoderm paleontology, Can. J. Zool., 79:1209–1231, 2001
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C. R. C. Paul and A. B. Smith (eds.), Echinoderm Phylogeny and Evolutionary Biology, 1988
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A. B. Smith, Echinoderm larvae and phylogeny, Annu. Rev. Ecol. Sys., 28:219–241, 1997
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A. B. Smith, Echinoid Palaeobiology, 1984
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alifazeli=egeology.blogfa.com
Additional
Readings
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D. B. Blake, A classification and phylogeny of post-Paleozoic sea stars (Asteroidea: Echinodermata), J. Nat. Hist., 21:481–528, 1987
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R. S. Boardman, A. H. Cheetham, and A. J. Rowell (eds.), Fossil Invertebrates, 1987
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R. A. Boolootian, Physiology of Echinodermata, 1966
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T. W. Broadhead and J. A. Waters, Echinoderms: Notes for a Short Course, 1980
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J. W.
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B. N. Haugh and B. M. Bell, Fossilized viscera in primitive echinoderms, Science, 209:653–657, 1980
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R. C. Moore (ed.), Treatise on Invertebrate Paleontology, pts. S, T, and U, 1966–1967
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C. R. C. Paul and A. B. Smith, The early radiation and phylogeny of echinoderms, Biol. Rev., 59:443–481, 1984
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Tree of Life Web Project
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