ON THE STKUCTURE OP HTDRACTINIA ECHINATA. 77 



On the Structure of Hydractinia echinata. 



By 



Iflarg-aret C. Collcutt, 



Zoological Laboratory, University College, London, 



With Plate 1. 



An investigation of the structure and relative positions of 

 the coenosarc and chitiuous parts of Hydractinia echinata 

 was suggested to me by Professor Weldon, for whose generous 

 advice and assistance I cannot be too grateful. 



From my observations on this hydroid I find that the chitin- 

 ous skeleton is for the most part a continuous irregular crust 

 attached to some foreign object, and overlaid by a coenosarc 

 consisting of two layers of ectoderm, enclosing between them 

 a number of branching and anastomosing endodermal tubes, 

 which are connected at intervals with the endodermal canals of 

 the polyps, the upper layer of ectoderm being continuous with 

 the ectoderm of the polyps. 



This view is somewhat similar to that of Strethill-Wright, 

 who describes the chitinous skeleton as forming a continuous 

 crust below the coenosarc, which, according to him, consists 

 of two ectodermal layers enclosing a single and continuous 

 layer of endoderm permeated by tubular cavities. 



With other observers of Hydractinia the prevalent idea 

 as to the structure of the colony seems to have been that the 

 skeleton was tubular, the chitinous tubes enclosing the coeno- 

 sarc, which also extended as a continuous sheet over the surface 



