284 MENSCH. [Vol. XVI. 



erable dimension, particularly in the region of the dorsal ten- 

 tacular cirrus, and its outline is more easily defined than in the 

 preceding stolon. More dorsally its outlines, as in the preced- 

 ing stolon, are lost in the external undifferentiated tissue which 

 constitutes what is here designated as the head. 



While the dorsal and lateral surfaces of the chain thus far 

 examined have undergone numerous changes by way of the 

 addition of new tissue and the formation of new structures, the 

 contour of the ventral surface has remained very much less 

 disturbed, and a ventral view of parts of stolons 2 and 3 (PI. XIII, 

 Fig. 9), in which dorsally great changes have taken place, indi- 

 cates comparatively little activity in the surface of this region, 

 the changes being confined to the addition of new segments of 

 similar character and not to the ventralward extension of dor- 

 sal thickenings. Such a view of this part of the chain would, 

 therefore, present a comparatively smooth surface ventrally, 

 with marked lateral irregularities caused by the difference in 

 size of the various segments. In this figure may be distin- 

 guished the exact outline of the anal segment {a.s.) of stolon 2, 

 the true size of its caudal cirri, and the position of this seg- 

 ment in relation to the head of the successive stolon. The 

 zone of separation {r.s) appears as a very narrow band of tis- 

 sue, the outlines of which are plainly visible in this view. 



Stolon 4 presents a still later stage in the process of devel- 

 opment. It consists of twenty distinct setigerous segments, 

 the last four or five of which are quite narrow as compared 

 with the preceding, and thus gives the outline of this stolon an 

 appearance quite different from that of the preceding ones and 

 makes it conform more closely to the outline of the free Poly- 

 bostricus. As compared with the preceding stolon, the seg- 

 ments of the anterior two-thirds of this stolon have increased 

 in breadth and a greater number of parapodia in an advanced 

 stage of development have become visible. The appearance of 

 one of the more advanced parapodia of a stolon of this age is 

 represented in PI. XIII, Fig. 10. At this stage the ventral branch 

 of the parapodium (v.r) has attained its full development and is 

 not unlike the lateral appendages which constitute the entire 

 parapodium in the parent stock so far as the position and num- 



