13.1 Introduction

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).

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. 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 upper or lower surface of the animal (oral surface), or at the anterior extremity. 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).

13.2 Classification

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.

13.2.1 Subphylum Homalozoa*

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.

13.2.2 Subphylum Pelmatozoa*

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.

Class Crinoidea*

• 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.

Class Asteroidea*

• 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.

13.3 Terminology
pentameral symmetry water vascular central disk anus spines pinnule ambulacral groove plates	bilateral symmetry colomic cavity podia regular echinoid Aristotle's lantern stalk spiracles interambulacrum	endoskeleton arms blind gut irregular calyx ambulacrum mouth madreporite