BY J AS. P. HILL. 15 



thicker in the middle — the continuation of the " keel " — derived 

 also from the throat epithelium as the opening of the notochordal 

 lumen gradually moved posteriorly (fig. 16, Ips'.). Towards the 

 posterior end of the skeleton the "keel" disappears entirely, tlie 

 two " nuclei " separate from each other ventrally, and the 

 epithelium of the throat extends up into the cleft between them. 

 The continuation of tlie body lying above the nuclei then thins 

 out, and the "nuclei" separate completely from each other, giving 

 rise to the " legs " proper which lie under a fold of the throat 

 epithelium. The "legs" may pass out at once almost transversely, 

 or they ma}- diverge more gradually in different individuals. 

 They terminate considerably in front of the middle region of the 

 collar. 



The proboscis skeleton is composed of a cuticular substance 

 which shows in this species a very distinct stratification, indicat- 

 ing the mode of origin of the mass by the deposition of successive 

 la3'ers, and the direction of these layers indicates very clearly 

 from what source they are derived. As Spengel has shown, the 

 "end plate" is derived from the ventral blind sac of the notochord, 

 while the " body" is derived from the neck of the same. The 

 " keel " on the other hand is formed from the epidermis of the 

 proboscis neck, and its posterior continuation from the epithelium 

 of the throat. 



In this species the " end plate " and the anterior portion of the 

 " body " are separated from the keel by chondroid tissue, and the 

 direction of the lines of stratification in these parts indicates 

 clearly enough their distinct origin. The relations of the skeleton 

 to the limiting membranes are essentially those of the described 

 species. The l^ody passes over at its edges into the limiting 

 membrane of the notochord, while the wings of the keel pass 

 over into that of the epidermis. 



In the proboscis skeleton of this si3ecies there occur a few 

 small oval cells with non-staining cell bodies and deeply staining 

 small nuclei. They are situated between the layers of stratifica- 

 tion. Such cells occur in the proboscis skeleton of Pt. clavigera, 

 yiyas and aurantiaca, and with Spengel I believe they are cells 



