DEVELOPMENT OP THE EXTERNAL FORM — HETEROPODA. 



157 



St.— 



-- S. 



J5 



showing it to be the principal part of the Gastropod foot. This is 

 evident from the fact that the fin originates at the anterior end of 

 the foot, the posterior side of the latter being covered by the 

 operculum (Figs. 66, 67) ; the intermediate part, i.e., the actual 

 rudiment of the foot, must therefore in any case be concerned in the 

 Formation of the sucker (Fig. 69 A), unless we are to regard the 

 latter as a secondary formation. The sucker usually appears much 

 later ; in Firoloida, it is only found in the male, and has therefore 

 here become merely a 

 sexual character. Here 

 and in Carinaria (Fig. 

 68), the sucker lies some- 

 what far down at the 

 margin of the fin, and 

 thus becomes absorbed 

 in the latter, its pedal 

 character being in this 

 way still more marked. 

 That the sucker is not 

 merely a supplementary 

 differentiation of the fin 

 is proved by forms such 

 as Oxyijyrus in which it 

 is independent of the fin 

 and lies behind the 

 latter (Fig. 69/1). We 

 have here great agree- 

 ment with the condition 

 of some Prosobranchs 

 (Rostellaria, Strombus, 

 Fig. 69 B), in which the 

 posterior part of the 

 foot, as the carrier of the 

 operculum, is sharply marked off from the anterior part. This view 

 corresponds on the whole with that adopted by Gegenbaur and 

 recently especially by Grobben (No. 38), as to the significance of 

 the foot in the Heteropoda. 



The tail found in the Heteropoda (Fig. 68, sw), also arises from the 

 foot, in Atlanta as a projection lying close behind the sucker (Krohn). 

 As it increases in size, this process, which also is cylindrical, presses 

 that part of the foot which bears the operculum more to the dorsal 



sc 



"Sg£y 



Fig. 69. — A, Oxyyyrus, B, Strombus, each viewed 

 from the side (after Sodleykt and Kiener). a, 

 eyes ; /, tentacle ; h, posterior part of the foot ; 

 op, operculum ; r, proboscis ; s, sucker ; sc, shell ; 

 sw, tail (most posterior section of the foot) ; v, 

 anterior part of the foot. 



