642 APPENDIX B 



Triploblastic. No marked metamerism. Adults radially symmetrical 

 (usually on a plan of five repeated parts). Inasmuch as larval echino- 

 derms are bilaterally symmetrical, the phylum has probably been derived 

 from bilaterally symmetrical ancestors. A large coelom and distinct 

 alimentary canal that usually, but not always, terminates in an anus. 



The following special features of the phylum are notable: skin usually spiny; 

 body wall with calcareous plates (much reduced in the sea cucumbers) that form 

 a protective exoskeleton; a peculiar water- vascular system that, in most forms, 

 constitutes a hydrostatic pressure system that regulates movements of the tube 



Fig. B.ll. Marine bryozoans or moss animals, greatly enlarged. On eel grass at left, a simply 

 branched creeping colony (Pedicellina) and an encrusting colony; on kelp at right, Mem- 

 branipora in large encrusting colonies. {Courtesy American Museum of Natural History.) 



feet (locomotor organs characteristic of this phylum). Circulatory and nervous 

 systems poorly developed. 



The echinoderms are all marine and constitute a considerable portion of the 

 animal life of the seashore. Among the objects most likely to attract the attention 

 of the shore visitor, particularly if the coast is somewhat rocky, are the five- 

 armed starfishes (class Asteroidea). These are often found in abundance, when 

 the tide is low, clinging to rocks and seaweeds. Among the seaweeds that grow 

 below the tidal zone there may be found somewhat similar animals called serpent 

 stars (class Ophiuroidea), which have five slender, wriggling, snakelike arms. Just 

 below the reach of low tide there occur other related animals, of rounded form, 

 covered with long, coarse spines; these are commonly known as sea urchins (class 

 Echinoidea). The flattened sand dollars are also members of this class. And if one 

 digs in the sand exposed between tides, he may encounter pink or whitish worm- 

 like animals, without distinct appendages but nevertheless able to cling to the 

 hand ; some of these belong to a group of the echinoderms known as sea cucumbers 

 (class Holothuroidea). The sea lilies, or feather stars (class Crinoidea), live mostly 



