86 ANIMAL BIOLOGY 



The Starfish is a member of the first class, the Asteroidea, of 

 the phylum, and serves to give some idea of the fundamental 

 anatomical plan of the several other classes. True, members of 

 most of the other classes bear little obvious resemblance to the 

 Starfish. Most similar are the Brittle Stars (class Ophiuroidea) 

 with flattened central disk and long, slender, and sometimes 

 branched arms that are fragile and readily discarded when injured. 

 But the Sea Urchins and Sand Dollars (class Echinoidea) are 

 without arms; the more or less spherical body being enclosed within 

 a hard shell composed of a multitude of closely fitting plates and 

 covered with a forest of spines. Then the Sea Cucumbers (class 

 Holothuroidea) are essentially elongated, flexible, muscular sacs 

 with contractile tentacles, representing modified tube feet, about 

 the mouth. They seem to be little inconvenienced if they eject 

 most of their internal organs, because a period of rest suffices for 

 their regeneration. And finally, the Sea Lilies (class Crinoidea), 

 apparently the antithesis of the Sea Cucumbers, are temporarily 

 or permanently attached, usually by a jointed stalk from which 

 extend their much-branched arms in plume-like fashion, and so 

 some are called Feather Stars. (Fig. 47.) 



Still, with all this diversified array of Echinoderms, it is, we 

 repeat, true that all are basically similar in structure and develop- 

 ment — they have almost surely been derived in a round-about 

 way from a primitive worm-like ancestor. 



H. Molluscs 



The great phylum Mollusc a, which includes not only such well- 

 known edible ' shell-fish ' as the Clams, Oysters, Scallops, and Snails, 

 but also the Cuttle-fish, Devil-fish, Nautili, etc., presents a consid- 

 erable departure in bodily plan from that exhibited by the Seg- 

 mented Worms, and constitutes another large and remarkable 

 branch of the Invertebrate ' tree. ' However, from certain structural 

 features exhibited by the lowest class, the Amphineura represented 

 by Chiton, and particularly the developmental stages of various 

 Molluscs, it appears clear that, like the Echinoderms, they have 

 arisen from a worm-like ancestral type which, instead of adopting 

 segmentation, became otherwise specialized to form a unique and 

 highly successful group. (Fig. 48.) 



The most characteristic structures of Molluscs are the external 

 skeleton, or shell; a fleshy muscular organ, the foot, typically 



