APPENDIX B 



643 



in deep water; they were more abundant in the past than they are today (Fig. 

 25.2). 



These five classes of animals, different as they may seem at first glance, are 

 included in this one phylum because they possess in common the features spe- 

 cified above. Certain indications from the developmental stages of echinoderms 

 suggest that these animals and the chordates had a common ancestry. 



Approximately 6,000 modern species of echinoderms are known. 



Phylum VIII. MOLLUSCA (mol his' ka; Latin, molluscus, "soft"). 

 Snails, clams, squids, and octopuses. Complex Metazoa, typically ex- 

 hibiting the following characters: 



Fig. B.12. Some echinoderms. A, a crinoid or sea lily, class Crinoidea. B, Arbacia, a sea 

 urchin, class Echinoidea. C, Asterias, a starfish, class Asteroidea. D, Ophiura, a serpent 

 star, class Ophiuroidea. E, a sea cucumber, class Holothuroidea. (Courtesy General Biological 

 Supply House, Inc.) 



Triploblastic. Bilateral symmetry, tending toward asymmetry (snails). 



Soft-bodied, unsegmented animals without jointed appendages. 



Coelom present. Complex respiratory (gills), circulatory, reproductive 

 (monoecious or dioecious) and nervous systems. Digestive system com- 

 plete with anus. Nervous system variable, not of the ladder or dorsal 

 cord types, exhibiting strong cephalization in certain groups that have 

 very complex organs of sight. 



Usually a shell of lime carbonate is present, secreted from the outer surface 

 of an enveloping layer of tissue called the mantle; a space, the mantle cavity, 

 separates the main body from the mantle. The shell may be: (1) of one piece, 

 pyramidal or coiled (in snails); (2) composed of two lateral valves (in clams, etc., 

 called bivalves); (3) composed of a series of small plates (in Amphineura); (4) 



