ON THE STRUCTUEE OF HYDEACTINIA EOHINATA. 



91 



The distal extremity is slightly enlarged into a club-shaped 

 tip, covered all over with nematocysts, which also occur in 

 considerable numbers throughout the ectoderm of the polyp. 



la examining colonies of Hydractiuia for these polyps I 

 came across two abnormal individuals. One of these had the 

 appearance of a tentacular polyp which had given rise to two 

 branches situated at different levels, and each terminating, like 

 the main polyp, in a rounded tip covered with nematocysts. 

 (Process, Fig. 1.) 



The other abnormality was a gasterozooid from which, just 

 below its tentacles, branched a tentacular polyp. (Process, 

 Fig. 2.) 



Fig. 1. 



Fig. 2. 



Histology. 

 A. Histology of the Lower Ectoderm. 



This layer extends, renews, and secretes the greater part of 

 the skeleton ; occasionally, in very limited areas, it is more 

 than one cell deep. 



The cells comprising it are not uniformly equal in size ; 

 beneath the polyps and towards the edge of the colony they 

 are more or less square and regular in shape, but in the adult 

 parts they are frequently deeper than they are broad. As a 

 gonsequence of the great variation in the depth of the cells of 



