372 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



stalks (y")' ^^^ *^® portion within the body and man- 

 tle {y'") . During its development the embryo has under- 

 gone an increase in size, and although the drawing is less 

 enlarged, the embryo shown in Fig. 197 is actually much 

 larger than that shown in Fig. 194. The external yolk- 

 sac shares in this growth, and is very much larger at a 

 somewhat later stage than the whole egg was at the begin- 

 ning of the process of development. 



Fig. 198 is a view of the posterior surface of an em- 

 bryo somewhat older than in Fig. 197. The external 

 yolk-sac (y) has grown so much larger that only a small 

 part of it is shown in this and the next three figures. The 

 mantle {m), has grown so much that the gills (^i'), and 

 the rectum are nearly contained in the mantle-cavity. A 

 constriction across the base of each gill has separated the 



branchial heart (h), from the gill 

 proper. The inner folds (si) of 

 the siphon, have united with each 

 other to form the closed siphon 

 tul)e, and the inner and outer 

 folds (si, si'), have met and are 

 unitins: with each other. 



gj Fig. 198. — The posterior surface of 



an older embryo, as seen from the right 



side, with the dorsal surface below. 



(Drawn from nature by W. K. Brooks.) 



For explanation of letters see Fig. 



195. 

 Fio. IflS. 



The walls of the otocysts, (er), have grown thin, and 

 their cavities have greatly enlarged ; the otoliths have 

 made their appearance, and the two chambers have begun 

 to move towards the median line, under the end of the 

 siphon. 



