THE ORDER AMONG ANIMAL TYPES 



III 



Class I. PELECYPODA (Fig. 51). Shell formed of two opposed 

 halves. Oysters, Clams, Mussels, etc., are common examples. 



MOLLUSCA 



FIG. 54. 



GASTROPODA-SNAIL 



FIG 53. AMPHINEURA-CHITON 



FIG. 55. SCAPHOPODA -TOOTH SHFLL 



FIG. 56. CEPHALOPODA- SQUID 



Figs. 53. 54, 55, and 56. — Representatives of four of the five classes of the phylum 

 Mollusca. See also Fig. 51 and Fig. 52. Although quite different in outward ap- 

 pearance, all have the essential features of the phylum, viz., mantle and mantle 

 cavity, ventral muscular foot, unsegmented bodies, and usually a shell. (Fig. 55 

 gready enlarged.) 



Class 11. AMPHiNEURA (Fig. 53). Deep marine forms. The shell 

 when present consists of eight transverse calcareous plates. This is 

 not a familiar form; Chiton is perhaps the most common example. 



