io8 



ELEMENTS OF BIOLOGY 



MOLLUSCOIDEA ECHINODERMATA 



BEAK 



FIG. 44. BRACHIOPODA {SHELL} 



FIG. 45. STARFISH 



ECHINODERMA TA 



ANUS 



DIGESTlve CANAL 



COELOM 



MOUTH 



FOOT 



WATER-yASCULAR SrSTEM 



FIG. 47. SEA URCHIN 



FIG. 46. DIAGRAM OF CENTRAL DISK AND 

 A RAY OF STARFISH 

 Fig. 44. — Brachiopoda, a representative of the second type of the phylum Mollus- 

 coidea. The calcareous shells of these animals much resemble shells of bivalve 

 MoUusca (Fig. 51). 



Fig. 45. — Echinodermata. Upper or dorsal view of a common starfish. 

 Fig. 46. — Diagram of the internal structure of the central disk and a single 

 ray of a starfish. Actually the stomach is very much larger than represented and 

 occupies most of the coelom. Note the tubular character of the digestive canal and 

 the radially arranged water vascular system. 



Fig. 47. — External appearance of the dorsal region of a sea urchin, a repre- 

 sentative of the phylum Echinodermata. Its internal structure is similar to that 

 shown in Fig. 46. 



