MONOGRAPH OF THE EXISTING CRIKOIDS. 467 



Corresponding to the transversely elliptical cross section of the vestibule, the 

 ventral side of the larva in the posterior portion is slightly flattened. Just over 

 the vestibule the cell layer is especially thin, and in the median line there are no 

 mesenchyme cells beneath the ectoderm. Thus the two borders of the ventral side 

 of the larvae stand out prominently on cross section, and one is easily tempted 

 to find in this evidence of the inception of the radial structure, though as a matter 

 of fact the future radii do not lie along these prominent borders, and originate 

 quite independently of them. 



In the next stage the vestibule has moved entirely within the calyx section 

 of the larva and has extended itself out over the greater part of the posterior end. 

 The ventral (outer) wall has become markedly thinner and shows a frail pave- 

 ment epithelium. It is entirely distinct from the mesenchyme. 



The hydroco?le ring has become noticeably wider, and an extensive, though 

 shallow, proliferation from the floor of the vestibule now entirely fills the central 

 space where earlier only a short peglike process was present. 



Throughout the whole floor the nuclei lie in several superposed layers, but 

 only in a few places can cell borders be demonstrated, and these almost never 

 can be followed through its entire thickness. 



In the succeeding stage the vestibule moves completely onto the posterior end 

 of the animal. It no longer extends over the hydroccele ring ventrally, while 

 dorsally it is already beginning to grow over the evaginations from the water 

 vessel the beginnings of the tentacles. 



The central proliferation extending through the water vascular ring has 

 enlarged noticeably, while it has grown anteriorly to before the hydroccele, its 

 advance being parallel to the posterior withdrawal of the end of the vestibule. 

 As this proliferation has migrated anteriorly with the gut. and in the meantime 

 has completely broken through, it is clear that the esophagus, passing through 

 the floor of the vestibule and the hydrocoele ring, is ectodermal in origin, uniting 

 itself with the stomach by means of the peglike process of the latter. 



In the fimnel-shaped esophagus, which is histologically very different from 

 the gut. cell borders can again be demonstrated through the entire thickness. In 

 spite of the apparent arrangement of the nuclei in several layers there can be shown 

 to be a single-layered epithelium of very high, slender prismatic cells the nuclei of 

 which lie near the middle, though in neighboring cells at different heights. 



The tentacle evaginations from the hydrocoele are already visible in the cavity 

 of the vestibule. 



The vestibule, having become more and more extensive, now includes the 

 entire posterior end of the calyx section. Its outer wall is cup shaped, the pos- 

 terior end (the base of the cup) rounded pentagonal, the anterior border (the 

 lip of the cup) where it goes over into the tentacle investment with at first very 

 strongly rounded angles and broader than the flattened posterior end. 



The lining is composed of pavement epithelium and lies closely upon the ecto- 

 derm layer, from which it is separated by sparse mesenchyme tissue. 



On the basal surface important alterations have taken place. The five evaei- 

 nations from the hydrocoele. at a time when they are scarcely perceptible within 



