THE MORPHOLOGY, STRUCTURE AND DEVELOPMENT OF . 

 HYDRACTINIA POLYCLINA. 



J. A. PLACE. 



With (lraiC'ln(/s hn J. T. Pickering. 



INTRODUCTION. 



During the summer of 1914, the Invertebrate Zoology class at the Marine 

 Biological Laboratory. Woods Hole, while working on Ei/ (J r actinia polyclina, 

 experienced some difficulty in finding, and identifying Tentacular polyps. The 

 question arose as to their constancy in the colony, and it was suggested by the 

 instructors as a good subject for special work. The purpose of this paper is 

 to offer a brief account of the morphology, structure, and development of the 

 various kinds of polyps that exist in a colony of Hydractinia polycUna. 



HABITAT. 



Hydractinia polyclina lives in a state of symbiosis with the Hermit-crab, 

 forming over the shell of the latter a soft, pinkish covering. It appears that 

 both are benefited by this community life, for while the Hydractinia colony is 

 furnished with transportation it, in turn, affords protection to the Hermit- 

 crab, not only by obscuring its shell from view, but also by its possession ot 

 stinging cells by means of which it forms a defense against the enemies of 

 the Hermit-crab. 



The constant association of these forms together led to the belief that 

 the life of the Hermit-crab was necessary to the existence of Hydractinia. 

 That Louis Agassiz found them growing in abundance, attached to rocks in 

 tide pools; that Samuel F. Clarke later found them growing on a wharf at 

 Fort Wool, Chesapeake Bay; that two students of Woods Hole Laboratory 

 found them growing on Mytilus edulis and Limulus ; that during the summer 

 of 1891 Dr. Conklin found them growing on the Fish Commission Wharf, 

 Woods Hole ; that they are frequently found growing on sponges about Woods 

 Hole region : have proved their existence independently of the Hermit-crab. 

 Besides having removed the occupants from shells bearing Hydractinia colo- 

 nies, we placed the shells in wire baskets and suspended them below low tide 

 from a wharf. There we left them three weeks, during which time the 

 colonies became very luxuriant on the empty shells. 



An additional advantage received by the Hydractinia colonies is that of 

 food supply furnished by the young paguri. Miss Bunting reports that many 

 of these are devoured by the polyps as they swim out from the maternal shell. 



21st MiOh. Acad. Sci. Rept., 1919. 



