MOLLUSC A. 



203 



which forms the shell-gland. On each side of the mantle is a some- 

 what curved fold ( /'). These folds eventually coalesce to form the 

 funnel. They are divided into two parts by a small body which forms 

 the cartilage of the funnel. The smaller part of the fold behind 

 this body gives rise to the true funnel, the part in front becomes 

 (Kolliker) the strong muscle connecting the funnel with the neck- 

 cartilage. In front and to the sides are two kidney- shaped bodies 

 (oc) the optic pits. Behind the mantle are two buds (6r), the 

 rudiments of the gills. 



FIG. 111. Two SURFACE VIEWS OF THE GERMINAL DISC OF SEPIA. (After KSlliker.) 

 mt. mantle; oc. eye; /. folds of funnel; br. branchiae; an. posterior portion of 

 alimentary tract ; in. mouth. 1, 2, 3,4, 5, arms; p. cephalic lobe. 



In the somewhat later stage rudiments of the two posterior pairs 

 of arms make their appearance outside and behind the rudiments of 

 the funnel. The head is indicated by a pair of lateral swellings on 

 each side, the outer of which carries the eyes. The whole embryo now 

 becomes ciliated, though the ciliation does not cause the usual rota- 

 tion. At a slightly later stage the second, third, and fourth pairs of 

 arms make their appearance slightly in front of those already present. 

 The posterior parts of the funnel rudiments approach each other, and 

 the anterior meet the rudiments of the neck-cartilage. The gills 

 begin to be covered by the mantle-edge, which now projects as a 

 marked fold. At a slightly later period two fresh rudiments may be 

 noted, viz. the oral (fig. Ill B, in] and anal imaginations, the la,tter 

 of which is extremely shallow and appears at the apex of a small 

 papilla which may be spoken of as the anal papilla. These imagina- 

 tions appear at the two opposite poles (anterior and posterior) of the 

 blastoderm. Shortly after this the rudiment of the first pair of arms 

 arises considerably in front of the other rudiments, at the sides of the 

 outer pair of cephalic swellings (fig. Ill B, i). 



Fig. Ill B represents a view from the dorsal surface of an embryo 

 at this stage. In the centre is the mantle with the shell-gland 

 which is now very considerably raised beyond the general surface. 

 Concentric with the edge of the mantle are the two halves of the 

 funnel, the anterior half meeting the dorsal or neck-cartilage and 

 the posterior halves approaching each other. The oral invagination 



