370 



HANDBOOK OF INVERTEBRATE ZOOLOGY. 



rhythmical waves of the outer layer are set in motion 

 on the surface of the yolk. The fact that the mouth is 

 dorsal to the row of arms at this period is worthy of 

 notice. 



The next figure (Fig. 196) is a view of the poste- 

 rior surface of a somewhat older embryo, represented, 

 with its dorsal surfVice beloAV. The mantle (m) now 

 overhangs the body considerably at the sides, as well 

 as posteriorly, and the portion of the yolk which pro- 

 jects into it is more sharply marked off than before, 

 and is drawn out to a point at the dorsal end. The e^^e- 

 stalks (es) and their yolk protuberances, are much more 

 prominent, and the constriction which separates the body 

 from the external yolk is much more marked. The three 

 pairs of arms are a little larger than before, and a cavity 

 is visible in each of them. The inner siphon-folds (si) 



have lengthened, and their outer 

 ends now point towards the outer 

 folds (si'), from which, however, 

 they are still widely separated. 

 The most important differences be- 

 tween this and the preceding stage 

 are differences of proportion and 

 a relative size, which are sufficiently 

 well shown in the drawings, and 

 do not call for description. 



Fig. 196. — The posterior aspect of an 

 older embryo, with its dorsal surface below. 

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

 Fig. 196. For explanation of letters see Fig. 195. 



¥ig. 197 is a posterior view of an older embryo, figured 

 with its dorsal surface below instead of above, in order to 



