MOLLUSCA. 



249 



is placed on the anterior side of the ventral protuberance, and 

 immediately dorsal to this is an invagination (os) which gives 

 rise to the stomodaeum. The ciliation at the edge of the blas- 

 topore still persists, but does not lead to the rotation of the 

 embryo. 



In later stages (fig. 113 C) the blastopore becomes closed, 

 and the mantle region increases in length as compared with the 

 remainder of the body. The ventral halves of the funnel, each 

 in the form of a half tube, coalesce together to form a single 



'Hit 



FIG. 113. THREE EMBRYOS OF A CEPHALOPOD WITH A VERY SMALL YOLK-SACK. 



(After Grenadier.) 



a. blastopore ; br. branchiae ; inf. 1 and inf." posterior and anterior folds of the 

 funnel ; g. op. optic ganglion (?) ; oc. eye ; wk. white body ; ac. auditory pit ; os. 

 stomodseum ; an. anus; tut. mantle; i, 2, 3. ist, 2nd, and 3rd pairs of arms. 



tube (inf) in the same manner as in Sepia. A shallow procto- 

 daeum (ait) is formed between the two branchiae. The eyes (oc} 

 stand out as lateral projections, and the arms become much 

 longer. 



Still later a fourth pair of arms is added as a bud from each 

 of the posterior pair, and with the growth in length of the arms 

 the suckers make their appearance. The mouth is gradually 

 carried up so as to be surrounded by the arms. The ciliation of 

 the surface becomes more extensive. 



During the whole of the above development the interior of 

 the embryo is filled with yolk, although no external yolk-sack is 



