MOLLUSC A. 225 



absence of a velum, (3) in the absence of a median foot, and in the 

 presence of the arms. 



The presence of a yolk-sack may most conveniently be spoken of 

 in connection with the foot, and we may therefore pass on to the 

 question of the velum. 



The velum is one of the most characteristic embryonic appendages 

 of the Mollusca, and its absence in the Cephalopoda is certainly very 

 striking. By some investigators the arms have been regarded as 

 representing the velum, but considering that they are primitively 

 placed on the posterior and ventral side of the mouth, and that the 

 velum is essentially an organ on the dorsal side of the mouth, this 

 view cannot, in my opinion, be maintained with any plausibility. 



Various views have been put forward with reference to the 

 Cephalopod foot. Huxley's view, which is the one most generally 

 adopted, is given in the following quotation 1 . 



" But that which particularly distinguishes the Cephalopoda is the 

 " form and disposition of the foot. The margins of this organ are, in 

 " fact, produced into eight or more processes termed arms, or brachia; 

 " and its antero-lateral portions have grown over and united in front 

 " of the mouth, which thus comes, apparently, to be placed in the 

 " centre of the pedal disk. Moreover, two muscular lobes which corre- 

 " spond 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 infundibidum." 



Grenacher, from his observations on the development of Ce- 

 phalopoda, 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 considerable modifications 

 in the course of the development. The foot is essentially a protu- 

 berance 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 Cepha- 

 lopods 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 



1 Tlie Anatomy of Invertebrated Animal*, p. 519. 

 B. E. 15 



