272 SUMMARY. 



" over, two muscular lobes which correspond with the epipodia of 

 "the Pteropods and Branchiogasteropods, developed from the 

 " sides of the foot, unite posteriorly, and, folding over, give rise to 

 " a more or less completely tubular organ the funnel or infun- 

 " dibulum" 



Grenacher, from his observations on the development of 

 Cephalopoda, argues strongly against this view, and maintains 

 that no median structure comparable with the foot is present in 

 this group : and that the arms cannot be regarded as taking the 

 place of the foot, but are more probably representatives of the 

 velum. 



The difficulty of arriving at a decision on this subject is 

 mainly due to the presence of the yolk-sack, which, amongst the 

 Cephalopoda as amongst the Vertebrata, is the cause of consider- 

 able modifications in the course of the development. The foot is 

 essentially a protuberance on the ventral surface, between the 

 mouth and the anus. In Gasteropods it is usually not filled with 

 yolk, but contains a cavity, traversed by contractile mesoblastic 

 cells. In this group the blastopore is a slit-like opening (vide 

 p. 187) extending over the region of the foot, from the mouth to 

 the anus, the final point of the closure of which is usually at the 

 oral but sometimes at the anal extremity. In Cephalopods the 

 position of the Gasteropod foot is occupied by the external yolk- 

 sack. In normal forms the blastopore closes at the apex of the 

 yolk-sack, and at the two sides of the yolk-sack the arms grow 

 out. These considerations seem to point to the conclusion that 

 the normal Gasteropod foot is represented in the Cephalopod 

 embryo by the yolk-sack, which has, owing to the immense bulk 

 of food-yolk present in the ovum, become filled with food-yolk 

 and enormously dilated. The closure of the blastopore at the 

 apex of the yolk-sack, and not at its oral or anal side, is what 

 might naturally be anticipated from the great extension of this 

 part. 



Grenacher's type of larva, where the external yolk-sack is 

 practically absent, appears to me to lend confirmation to this 

 view. If the reader will turn to fig. 113, he will observe a promi- 

 nence between the mouth and anus, which exactly resembles the 

 ordinary Gasteropod foot. At the sides of this prominence are 

 placed the rudiments of the arms. This prominence is filled 



