206 



CEPHALOPODA. 



ments in Sepia. The eye has undergone considerable changes. 

 Close to each rudiment of the funnel may be seen a fresh sense- 

 organ the auditory sack (ac). The ventral (upper in the figure) 

 end of the body now forms a marked protuberance, probably equi- 

 valent to the foot of other Mollusca (vide p. 225), at the sides of 

 which are seen the rudiments of the arms (1, 2, 3). To the two 

 previously present a third one, on the posterior side, has been added. 

 The blastopore is placed on the anterior side of the ventral protu- 

 berance, and immediately dorsal to this is an invagination (os) which 

 gives rise to the stomoda3um. The ciliation at the edge of the 

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

 embryo. 



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



rnt 



B 



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



(After Greuacher.) 



a. blastopore; fcr. branchias ; in/. 1 and inf. 2 posterior and anterior folds of the 

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

 os. stomodaeum ; an. anus; mt. mantle; 1, 2, 3. 1st, 2nd, and 3rd pairs of arms. 



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 tube (inf) in the same 

 manner as in Sepia. A shallow proctodaeum (an] 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 present. 



