MOLLUSCA. 273 



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 over- 

 looked 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 unre- 

 presented. 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. 



The epiblast of the mantle has the special capacity of secret- 

 ing a shell, and the integument of the foot has also a more or 

 less similar property in that it forms the operculum, and a 

 byssus in some Lamellibranchiata, other parts of the integument 

 form the radula, setae in Chiton, and other similar structures. 



Nervous system. The origin of the nervous system in 

 Mollusca is still involved in some obscurity. It is the general 

 opinion amongst the majority of investigators that the nervous 

 ganglia in Gasteropods and Pteropods are formed from detached 

 thickenings of the epiblast. Both Lankester (No. 239) and Fol 

 (No. 249 251) have arrived at this conclusion, and Rabl has 

 shewn by sections that in Planorbis there are two lateral thick- 

 enings of the epiblast in the velar area ; from which the supra- 



B. II. 18 



