NERVOUS SYSTEM. 



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. 



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

 tically 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 Grenadier and Huxley agree in regarding the funnel as 

 representing the coalesced epipodia ; but Grenadier points out that 

 the anterior folds which assist in forming the funnel (vide p. 210) 

 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 svstem. 



' 



The epiblast of the mantle has the special capacity of secreting 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 thickenings of the epiblast in the velar area; from 

 which the supra-oesophageal ganglia become subsequently separated 

 off. The observations on the pedal ganglia are less precise : they 



