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Phylum Echinodermata - starfish, urchins, sea cucumbers, sea lilies

• ~7000 species.

• Marine.

Commonly echinoderms are conspicuously spiny, especially the Echinoidea, hence the scientific name from the Greek echinos meaning spiny and derma meaning skin. Unique to the echinoderms is their water vascular system. This series of canals employs hydrostatic pressure and ciliated cells to direct water to the podia or tube feet which are used in locomotion or feeding. Typically the system is open to the surrounding waters via the madreporite which is connected to the ring canal. Another feature of the echinoderm body is that it is radially symmetrical in five planes (pentaradial) radiating from the mouth. The internal skeleton is comprised of calcified plates called ossicles which in most echinoderms form a protective shell around the internal organs.

Class Crinoidea - sea lilies, feather stars

• ~650 species

• Marine

There are 2 main groups of crinoid (Figure 33) based on whether the calyx (equivalent to the central disc) is in contact with the surface via a flexible stalk (Isocrinida or feather stars) made up of calcareous discs (columnals) or via prehensile cirri (Commatulida or sea lilies) which enable the animal to walk or anchor. The mouth and anus are directed upward from within the membranous tegmen, which is supported by the cup-shaped calyx. The arms extend from calyx in multiples of 5, each with 2 rows of tubular pinnules along their length. The ambulacral grooves are flanked by tube feet (in triplets) near the base of the pinnules. The tube feet excrete mucous to trap food in the water column and then pass items down to the mouth.

The Isocrinida are typically found in deep water, and the Commatulida are more common in shallow water. Both groups are capable of locomotion, for instance the isocrinids use the arms to lift themselves from the substratum and crawl. As mentioned above the Commatulida are capable of walking using the cirri but also swimming short distances using powerful downward strokes of the arms.

Class Stelleroidea - starfish and brittlestars

• ~4,000 species

• Marine

This class contains two subclasses, the Ophiuroidea and Asteroidea, both of which bear arms or rays that extend from a central disc region. This often leads to difficulty in distinguishing the groups by the casual observer. The two classes are, however, strikingly different in a number of features in both the adults and larval stages.

Sublcass Ophiuroidea - brittlestars

• ~2,000 species

• Marine

Ophiuroids (Figure 34) are similar to the asteroids in appearance but with some very important differences. The central disc is very well defined and always has 5 arms, although the arms may be extensively branched, as in the basket stars (gorgonocephala). The arms are very flexible (and very fragile) and are used to push and pull the animal along as well as capturing food. The arms (Figure 36) do not have an ambulacral groove and the tube feet lack suckers and ampullae, playing no part in locomotion but are instead used to pass food to the mouth. Internally the arms do not house the gonads or gut, but are mostly taken up by muscles and vertebral ossicles, giving the arms their flexibility. The five madreporites are on the oral shields and connect to the ring canal.

Sublcass

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