PHYLUM ECHINODERMATA
Echinoderms include common seashore animals such as seastars (also known as
"starfish"), sand dollars and sea urchins, along
with hundreds of more exotic forms. Their basic body plan is very different from
other animals, but their closest living relatives
are the Phylum Chordata (which includes the vertebrates). Although they are marine animals, they are not closely related to the
Cnidarians due to the fact that Cnidarians DO NOT have endoskeletons.
Echinoderms are exclusively marine, and most are benthic. They are present in
virtually all marine environments of normal
salinity, from the shallow intertidal to the abyssal zone. Many echinoderms are
suspension feeders, while others are predators,
scavengers and herbivores. A few are deposit feeders.
Although the phylum is quite diverse, echinoderm physiology and their body plan
display a surprising uniformity. They are
characterized by an internal skeleton (endoskeleton) composed of calcitic plates
(ossicles), and a water vascular system. The
ossicles have a porous microstructure that is distinctive. A major feature of
the skeleton is that the ossicles may increase in size
during the growth of the animal. This feature is what scientists use as their basis for classifying them closely with the chordates.
The main portion of the body skeleton, known as the theca or calyx in most
echinoderms, may have accessory appendages
(arms, rays, stem or brachioles).
The water vascular system is an interesting system unknown in any other phylum.
In ancient echinoderms, water circulated
through pores in the body wall and was apparently important for respiration and
feeding. More derived taxa have a specialized
system where the water is drawn through a sieve plate (madreporite) by the
action of cilia or internal pumping. The water enters
a calcified tube and is directed to various parts of the animal. The water
eventually fills small sacs inside external tube-like
extensions (the tube feet or podia) along the rays and these, through hydraulic
manipulation, may pulsate to move the animal
through the environment or transport food to the mouth.
Echinoderms are generally radially symmetric, with adults displaying a secondary
pentaradial symmetry. The symmetry is
secondary, because echinoderm larvae are bilaterally symmetric. One group, the
sea cucumbers, develop a tertiary bilateral
symmetry. The mouth is located centrally on the lower surface of the animal (oral surface). The other surface is termed the aboral surface.
A coiled gut extends from the
mouth to an anus, which is situated between two
rays or at the posterior end. Echinoderms have a well-developed nervous system
and reproductive system, but no heart (no
need with the water vascular system).
Phylum Echinodermata contains over a dozen classes, about half of which are
known only from the Paleozoic. They are
classified by characters such as the general morphology, ossicle structure,
arrangement of the water vascular system, and
embryology.
The homalozoans include the "carpoid" echinoderms and possibly another minor
group. Carpoids are small and rare fossils
found only in Lower and Middle Paleozoic rocks. They have an asymmetric,
flattened body composed of calcitic plates, and a
short stem called an aulacophore. Carpoids have been assigned to the
Echinodermata because the calcite of their plates has a
characteristic echinoderm microstructure, and because most bear a food groove of
some type.
Pelmatzoans are ehinoderms that are radially symmetrical to some degree, have a
generally cupshaped body (theca) enclosing
the viscera, and possess food-gathering appendages (arms or brachioles)
extending from the theca. Most pelamtozoans have a
jointed stem that is usually used to attach the animal to the substrate.
Only this group of pelmatozoans is not confined to the Paleozoic. Crinoids are
the most diversified and common
members of the subphylum-in some Paleozoic deposits their scattered ossicles
form the bulk of the rock. The typical
crinoid has a long stem with "roots" or some other attachment device (holdfast)
at the lower end, and a cup-shaped
thecum at the top. Several arms extend from the dorsal surface of the theca to
collect suspended food. These arms are
commonly branched and bear calcitic columnals very similar to those of blastoids
and cystoids. Some crinoids have lost
the stem and become mobile. Recent crinoids are found mostly at bathyal depths,
though some are common in tropical
reef and shallow cave habitats. They range from polar to tropical latitudes. The
Paleozoic forms were very common on
shallow carbonate platforms. Four subclasses were recognized, only one of which
(the Articulata) is extant today.
This ancient group is widely distributed in Early and Middle Paleozoic rocks.
The cystoid theca is usually globular or
saclike, with radial or, rarely, bilateral symmetry. It is made of numerous
plates, regularly or irregularly arranged, that are
pierced by a characteristic pore system. The stem, or column, is made of
numerous circular or elliptical plates, known as
columnals. Not all cystoids had a column, and many apparently did not use it for
attachment. The feeding system of a
cystoid consisted of a set of brachioles, which were free appendages on the
dorsal surface used for gathering suspended
material. The class is divided into two main groups based on the structure of
the thecal pores.
The blastoids were stemmed forms with pronounced pentaradial symmetry and a very
regular distribution of thecal
plates. The theca is the most commonly preserved part of the animal- it has the
general shape and approximate size of a
rosebud. The majority of blastoids have 13 thecal plates arranged in three
circlets. The ambulacral areas are specialized
and bear numerous food-gathering brachioles. All blastoids have a distinctive
structure called the hydrospire that hangs
into the body cavity beneath each ambulacrum; it is thought to have functioned
as a respiratory device.
These are the typcial seastars found along the seashore. They generally have
hollow arms, into which the coelomic cavity
exends; the radial canals are located on the outside of the skeleton. Asteroids
crawl by concerted actions of their podia,
and so are very sluggish movers. Most are carnivorous, feeding on all types of
benthic animals. Clams are the most
common food - some species can force the valves apart and extrude their stomachs
into the shell to digest the meat.
Gastropods and cnidarians are other favourite items. The lower side of each arm
contains an ambulacral groove with
open radial water vessels and numerous large podia.
In contrast to the asteroids, the ophiuroids are active crawlers with whip-like
arms that wriggle like snakes (hence they
are sometimes called serpent stars). They have a robust central disc that
contains all the viscera. The arms are not
hollow, but are filled with a series of articulating ossicles resembling the
vertebrate backbone. Ophiuroids are also called
brittle stars because of their habit of releasing arms at ossicle sutures to
escape predation or other antagonistic
behaviours. This process of releasing a limb is called autotomy. The lost limb
is eventually regenerated.
Many ophiuroids are deposit feeders, while some capture suspended food or small
prey with their podia (suspension
feeding). The mouth is centrally located on the lower side and leads to a blind
gut with no anus.
These
echinoderms are found throughout the Paleozoic, although they are usually rare.
They have a saclike to discoidal
theca made of
many irregular plates. Usually five straight or curved ambulacral areas extend
from the mouth, which is
located
centrally on the upper surface. Most edrioasteroids attached themselves to some
firm object, such as a shell or
rock. This
extinct class now includes cyclocystoids.
This group includes the sea urchins (regular echinoids), heart urchins (spatangoids),
and sand dollars (clypeasteroids).
Echinoids are usually globular, discoidal or heart-shaped and have a skeleton
made of many calcitic plates. The skeleton
has five ambulacral areas, each numerously perforated for the tube feet, and
five interambulacral areas, which bear
spines. Echinoids have a unique jaw apparatus known as Aristotle's Lantern that
is protrusible from the mouth. The most
ancient echinoids have the anus (with its set of enclosing plates, the periproct)
located in the center of the aboral surface
(the "regular" condition). Some later forms show a gradual migration of the
periproct toward the border (the "irregulax"
condition).
Regular echinoids can be distinguished easily from irregular echinoids by their
circular test, nearly perfect pentameral
symmetry, and the central location of the anus (directly above the mouth). The
ambulacra have 2 or more columns of
plates. The interambulacra have one or more columns of plates and are all
similar. The spines are generally long and an
Aristotle's lantern occurs in all taxa. All Paleozoic echinoids were regular.
Irregular echinoids are distinctively elongate in the adult stage. This shape
difference as well as the posterior position of
the anus (instead of dorsally, like the regular echinoids) are the 2 most
telltale differences setting the two types apart.
Irregulars also usually have petals on the upper surface, and each ambulacrum
and interambulacrum has 2 columns of
plates (with the exception of the posterior ambulacrum, which differs from the
others). Spines are generally short and
Aristotle's lantern is absent in most adult forms, except for the sand dollars.
The irregulars underwent a spectacular
radiation in the Mesozoic and are much more common as fossils, compared to the
regulars. The derived irregulars also
have concentrated the respiratory devices on the aboral surface, and have
developed food grooves.
These are the sea cucumbers, which do not superficially resemble any of the
other echinoderms. Close examination however
reveals that they do have a pentaradial symmetry, but the anus is opposite the
mouth on an elongated oral-aboral axis. The
calcitic plates are reduced to dermal, microscopic sclerites, which are often
used in classification schemes. They have a water
vascular system and podia. Holothurians are generally deposit feeders- they use
small tentacles surrounding the mouth for
particle collection. Several species are suspension feeders. A few rare forms
are planktonic.
