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[111].
“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 anterolateral 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 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 infundibulum.”
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 considerable 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 prominence 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 with yolk, and represents the rudiment of the external yolk-sack of the typical Cephalopod embryo. The blastopore, owing to the smaller bulk of the food-yolk, reverts more nearly to its normal position on the oral side of this prominence.
If the above considerations have the weight which I attribute to them, the unpaired part of the Cephalopod foot has been overlooked in the embryo on account of the enormous dilatation it has undergone from being filled with food-yolk; and also owing to the fact that in the adult the median part of the foot is unrepresented. The arms are clearly, as Huxley states, processes of the margin of the foot.
Both Grenacher and Huxley agree in regarding the funnel as representing the coalesced epipodia; but Grenacher points out that the anterior folds which assist in forming the funnel (vide p. [253]) represent the great lateral epipodia of the Pteropod foot, and the posterior folds the so-called horse-shoe shaped portion of the Pteropod foot.
Development of Organs.
The epiblast. With reference to the general structure of the epiblast there is nothing very specially deserving of notice. It gives rise to the whole of the general epidermis and to the epithelium of the organs of sense. The most remarkable feature about it is a negative one, viz. that it does not, in all cases at any rate, give rise to the nervous system.