MOLLUSCA. 



279 



is provided with two rows 

 of cells, bearingon 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 de- 

 rived 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 spin- 

 dle muscle attaching the 

 embryo to the shell. This 

 muscle appears to be absent 

 in the Cephalopoda. 



Body cavity and vas- 

 cular system. The body 

 cavity in Gasteropods and 

 Pteropods originates either 

 by a definite splitting of 

 the mesoblast, or by the ap- 



rt 



ins 



FIG. 125. SECTIONS THROUGH THE DE- 

 VELOPING EYE OF LOLIGO AT TWO STAGES. 

 (After Bobretzky.) 



hi. inner segment of lens ; vl, outer segment 

 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; ms. layer of mesoblast between the two 

 epiblastic layers of the ciliary body; af. and 

 if. fold of iris ; rt. retina ; rt" . inner layer 

 of retina; st. rods ; aq. equatorial cartilage. 



pearance of intercellular 



spaces. It becomes divided into numerous sinuses which freely 



communicate with the vascular system. 



Very different accounts have been given by different investi- 

 gators 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 estab- 

 lished this mode of development for Nassa. 



