XVII 



PEOTOCHOEDATA 



627 



stigmata, increase in a remarkable manner at the time of meta- 

 morphosis (Fig. 453, C). On each side, each of the slits, at its ventral 

 end, curves inwards towards its fellow of the same side, so that each 

 takes on the form of a U. Then by a downgrowth from the dorsal 



Fig. 456. — Transverse sections througli the hinder part of the pharynx of a series of 

 larvae and just-fixed young of the simple Ascidian Oiona intestinalis, in order to 

 illustrate the development of the heart pericardium and epioardium. (After Kuhn.) 



A, through free-swiraming tadpole — the pericardium arises as a thickening of the ventral pharyngeal 

 wall. B, through free-swimming tadpole— a later stage ; the pericardium forms a sac opening into the 

 pharynx. C, a still later stage (still free-swimming) — the pericardial vesicles cut off from the pharynx. 

 D, through a just-fixed tadpole — the heart arises as a fold of the dorsal wall of the pericardium. E, 

 through an older specimen than that shown in D — the heart has become hollow and contains blood 

 cells. F, through a still older specimen — the epicardial tubes arise as independent evaginations of the 

 pharynx above the pericardium, ep.l, left epicardial tube ; ep.r, right epicardial tube ; //, heart ; h.f, 

 fold which gives rise to the heart ; pli, pharynx ; per, pericardium ; si, stomach. 



wall, each U becomes divided into two slits, and so two new slits are 

 intercalated between the first two, which for this reason we numbered 

 No. 1 and No. 4 when we described them above. This process 

 reminds us of the division of the primary slits in Amphioxus by the 



