IX 



MOLLUSCA 



36: 



completed. The anterior chamber of the eye is formed as an ecto- 

 dermic fold, which arises from the base of the eye-stalk and encloses 



me. 



H 



-tic, 



it. In this way the 

 outer surface of the adult 

 head is formed and the 

 eye-stalks are no longer 

 prominent. This en- 

 veloping fold we may 

 term the corneal fold. 

 From the spot where the 

 primary eye vesicle closed 

 the rudiment of the outer 

 segment of the lens is 

 formed as an external 

 secretion. The eyelids P er 

 are constituted by a pair 

 of folds similar to those 

 which wall in the anterior 

 chamber, but lying out- 

 side the latter. Lying- 

 inside the anterior 

 chamber is still another 

 pair of folds which con- 

 stitute the iris. 



The alimentary canal, 

 which had become a 

 completed tube in the 

 previous period, now 

 shows further develop- 

 ment. The radula sac 

 becomes sharply marked off from the stomodaeum. In front of it two 



a. 



/,- 



brh 



m.c. 



FIG. 295. Diagrams of three transverse suctions 

 through an emliryo of Ln/fi/n nil</ttris, much older 

 than that represented in Fi-. 'J'.M. (After Fanssi-k.) 



A, The most posterior; C, the most, anterior of (lie three 

 sections. Letters as in jnwions iimne. In addition, aur, 

 auricle; /,-, kidnr\ ; ,.,-, mantle-cavity; m.r. mant I^-VIMII ; 

 //, pericardium. 



