CORYNID^ : HYDRACTINIA. 35 



together, " is about the one-twentieth of an inch in thickness. 

 When first taken out of the water, it is soft and spongy, but be- 

 comes rigid on drying ;" and, in this state, the surface is muricated 

 with papilla?, about a line in height, and rough with minute prickles 

 pointing upwards. 



It was long believed that the connection between this muricated 

 crust and the soft polypes was only accidental. In this country 

 Mr. Hassall was the first to afiirm the contrary. His observations 

 appeared to me insufficient to prove their constant and organical 

 connection, and Professor Edw. Forbes contended strongly for the 

 older view. It is now unnecessary to enter into the discus- 

 sion, for the correctness of Mr. Hassall's conclusion is proved by 

 the prior observations of Van Beneden, and by the more recent 

 researches of Philippi and M. de Quatrefages ; for I have no doubt 

 whatever of the sameness of the species these various naturalists 

 have described. It is very true, that, in Van Beneden's excellent 

 figure, the crust is represented as being even and smooth ; but 

 there are scattered remarks in his reply to M. de Quatrefages, which 

 imply that it was actually rough ; and we know, that when fresh 

 and living, the crust, which appears so rough when dried, is covered 

 with an organic slightly diaphanous jelly, which almost conceals the 

 murications, and beyond which they scarcely project. 



The shells which Hydractinia echinata infests, are usually te- 

 nanted by the Hermit-Crab ; and I have often noticed the rim of 

 the aperture of the shell to be extended by the growth of the 

 zoophyte in the form of a horny membrane, by w^hich the capacity 

 of the crab's domicile was much enlarged. Mr. Gray has made 

 similar observations. He says, " This Alcyonium gradually enlarges, 

 and, being moulded on the body of the Hermit-Crab, forms for that 

 animal a house adapted to its growth, so that it has no necessity either 

 to enlarge its house by the absorption of part of the cavity of the shell 

 which it inhabits, or by leaving the shell, and seeking for another 

 better adapted to its size, as other specimens are obliged to do, which 

 have not the assistance of the coral. One can understand that the 

 crab may have the instinct to search for shells on which the coral 

 has begun to grow ; but this will scarcely explain why we never 

 find the coral except on shells in which Hermit-Crabs have taken up 

 their residence." 



I am almost persuaded that H. echinata is the same species as the 

 Hydra cainiata of MUller, Zool, Dan. prod. no. 2790 ; and that his 

 //. hrevicornis is the same animal in its young state, when only 

 eight ten taenia have been developed. d 2 



