ON THE STRUCTURE OF UYDKAOTINIA EOHINATA. 83 



as the hermit-crab drags its home about ; a small colony is 

 usually situated uear the edge of the shell. 



A Hydractinia colony is comprised of four kinds of polyps : 



1. Gasterozooids, or Nutritive polyps. 



2. Blastostyles, or Reproductive polyps. 



3. Dactylozooids, or Spiral polyps. 



4. Tentacular polyps. 



The Gasterozooids are the most numerous of the polyps ; in 

 the early spring and in the summer, however, the Blastostyles 

 increase greatly in numbers, and at these times give rise to 

 the generative products, the colonies being either male or 

 female. 



Round the shell mouth are situated the Dactylozooids, 

 which are capable of coiling themselves spirally, and may 

 function as defensive polyps. 



Strethill- Wright (10) and Hincks (12) also mention "Tenta- 

 cular polyps/^ which are scattered on the outskirts of the 

 colony, and which they regard as constant in occurrence. 

 Allman (13), on the other hand, does not believe that they 

 are universally present, and supposes that they are merely 

 abnormalities when they do occur. 



However, in all those colonies which I have closely exa- 

 mined with a view to finding these polyps I have been 

 successful; in one specimen they were particularly abundant, 

 occurring principally at the growing edge of the colony. In 

 other specimens they were rare, while on one very flourishing 

 colony I only found about half a dozen. I doubt if they are 

 abnormal dactylozooids, as Allman suggests, for I have never 

 found them situated on that part of the colony frequented 

 by dactylozooids. 



These various kinds of polyp are all connected together at 

 their bases by a common coenosarcal expansion, which is a 

 white semi-transparent investment completely covering the 

 chitinous skeleton. 



I succeeded in keeping several colonies alive for some time 

 by suspending the shells on which they were situated in small 

 tanks, the other occupants of the shells being removed. One 



