20 HYDRACTINIID^E. 



and (5) the tentacular filaments, highly extensile thread- 

 like processes with nematocysts at the tip, distributed 

 chiefly on the outskirts of the colony. In Podocoryne we 

 have, as a variation, a locomotive sexual zooid. 



In studying these remarkable organisms, we are at once 

 reminded of the oceanic Hydrozoa, the complex colonies 

 of which float freely in the open sea. There are many 

 striking resemblances between Hydr actinia and some of 

 the Physophoridas. Both exhibit the same polymorphism ; 

 in both the reproductive bodies are borne on peculiarly 

 modified polypites; in both tentacular appendages are deve- 

 loped from the ccenosarc, and a solid expansion supports the 

 community. Hydractinia and Podocoryne, from the nature 

 of the habitat which they almost invariably select, enjoy 

 the benefits of locomotion, though themselves fixed. They 

 employ the mollusk and the Hermit-crab as their carriers, 

 and to some extent, probably, as their purveyors also. 



The expanded crust of Hydractinia supporting the 

 curious assemblage of zooids has been investigated by 

 several eminent naturalists, from whom we have had con- 

 flicting accounts of its nature and the mode of its forma- 

 tion. Agassiz takes the view that the whole horny mass 

 is a " foot-secretion " just as truly as it is among the 

 gorgonioid polyps an opinion which had been previously 

 maintained by De Quatrefages. Dr. Strethill Wright, 

 who has thoroughly investigated the history of Hydrac- 

 tinia, and who was the first to notice some of the most 

 interesting points of its structure, arrives at the opposite 

 conclusion, that the mode in which the polypary is secreted 

 is essentially the same as amongst other hydroid zoophytes. 

 There is some difficulty in examining the chitinous 

 expansion, closely adnate, as it usually is, to the body on 

 which the colony is planted. Frequently, however, in the 

 case of old shells tenanted by the Pagurus, it is found to 



