328 



GEOLOGICAL BIOLOGY. 



tive moUusk. In the Gastropoda anisopleura, or typical Gas- 

 tropods, the specialization does not greatly affect the foot, 

 which is still symmetrical and occupies similar relations to 

 the rest of the body ; but the twisting of the body coincident 

 with the spiral shell which is developed as a cover, affects the 

 proportionate size and vigor of the organs on the two sides, 

 so that the organs are in fact not strictly symmetrical in the 



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Fig. loi. — Diagrams of a series of Mollusks 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). .-J, /", antero-posterior horizontal 

 axis ; Z*, K, dorso-ventral vertical axis at right angles to A^ P\ o, mouth ; a, anus; ftis, edge 

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

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



adult (3). The Pteropods have the foot modified for free 

 swimming into two lateral flappers or wings (Fig. loi, (4)), and 

 the Cephalopods proper have the right and left lobes, corre- 

 sponding to the wing-expansion of the Pteropods, folded 

 under to form a funnel-like tube or siphon, which accom- 

 plishes locomotion by forcing water outward and forward, the 



