MOLLUSC A. 



On the side opposite Kolliker's duct, an epithelial ridge is formed- 



the crista acustica the cells 

 connected with the crista by 

 a granular material. At a 

 later period of development 

 three regions of the epithelium 

 of the sack become especially 

 differentiated. Each of these 

 regions is provided with two 

 rows of cells, bearing on their 

 free edges numerous very short 

 auditory hairs. The cells of 

 each row are placed nearly at 

 right angles to those of the 

 adjoining row. 



Muscular system. The 

 muscular system in all groups 

 of Molluscs is derived entirely 

 from the mesoblast. 



The greater part of the 

 system takes its origin from 

 the somatic mesoblast. In 

 almost all Gasteropod and 

 Pteropod larvae there is pre- 

 sent a well-developed spindle 

 muscle attaching the embryo 

 to the shell. This muscle ap- 

 pears to be absent in the 

 Cephalopoda. 



Body cavity and vascular 

 system. The body cavity in 

 Gasteropods and Pteropods 

 originates either by a definite 

 splitting of the mesoblast, or 

 by the appearance of intercel- 

 lular spaces. It becomes di- 

 vided into numerous sinuses 

 which freely communicate with 



if which 



give 



rise to an otolith 



r/ 



A. 



B 



FIG. 125. SECTIONS THROUGH THE DE- 

 VELOPING EYE OF LOLIGO AT TWO STAGES. (After 



Bobretzky). 



hi. inner segment of lens ; vl. outer seg- 

 ment of lens ; a and a', epithelium lining the 

 anterior optic chamber ; gz. large epiblast cells 

 of ciliary body ; cc. small epiblast cells of 

 ciliary body ; JHS. layer of mesoblast between 

 the two epiblastic layers of the ciliary body ; 

 /. and if. fold of iris; rt. retina; rt". inner 

 layer of retina ; st. rods ; aq. equatorial carti- 



the vascular system. 



Very different accounts have been given by different investigators 

 of the development of the heart in the Gasteropoda and Pteropoda. 



It would seem however in most cases to arise as a solid mass 

 of mesoblast cells at the hind end of the pallial cavity, which 

 subsequently becomes hollowed out and divided into an auricle 

 and ventricle. Bobretzky's careful observations have fully established 

 this mode of development for Nassa. 



Iu Pteropods the heart is formed (Fol) close to the anus, but slightly 



