250 



GEOLOGICAL BIOLOGY. 



class Cephalopoda, with two subclasses, (i) the Pteropoda and 

 (2) the Siphonopoda. 



The classes are chiefly distinguished by modifications of 

 the foot, as is beautifully shown in Fig. 57. 



ff trif 



Fig. 57. — Diagrams of a series of Moll jsks to show the form of the foot and its regions, and the 

 relation of the visceral hump to the antero-posterior and dorso-ventral axes, (i) A Chiton. 

 (2) A Lamellibranch. (3) An Anisopleurous Gastropod. (4) Thecosomatous Pteropod. (5) A 

 Gymnosomatous Pteropod. (6) A Siphonopod (Cuttle). W, /', antero-posterior horizontal 

 axis; Z>, F, dorso-ventral vertical axis at right angles lo A^ P\ o, mouth ; a, anus; nis, edge 

 of the mantle-skirt or flap; i/, sub-pallial chamber or space; ^, fore-foot: «//", mid-foot; 

 hy, hind-foot ; e, cephalic eyes; cd, centro-dorsal point (in 6 only). (After Lankester.) 



In the Gastropoda the foot is simple, median in position, 

 and flattened so as to form a broad, sole-like surface (No. 3). 



In the Scaphopoda the foot is adapted to burrowing life 

 in the sand. 



In the Pteropod the mid-foot is developed laterally into 

 paddle-like swimming organs, and the fore-foot may be spe- 

 cialized into tentacles (Nos. 4 and 5). 



In the Cephalopoda the fore, middle, and hind foot parts 

 are separately specialized, the fore-foot merging with the head 

 part and developing into arm-like processes, in some cases 



