24 HYDRA CTINIID^E. 



serrated spines; ALIMENTARY POLYPITES milk-white, with 

 a variable number of tentacula (20-30 in*. the adult), 

 which are held in extension, alternately elevated and 

 depressed; FERTILE POLYPITES short and slender, bearing 

 the gonophores in clusters or scattered upon the upper 

 portion of the body; GONOPHORES (male) oblong and 

 pointed above, of a yellowish colour, (female) roundish 

 and rose-coloured, occasionally developed on the com- 

 mon base ; APPENDAGES OF THE CCENOSARC, long, filamen- 

 tary organs spirally coiled while at rest, with clusters 

 of thread-cells round the free extremity, and slender, 

 very extensile tentacula distributed singly 011 the out- 

 skirts of the colony. 



H. ECHINATA selects for its habitat invariably, so far as I 

 have observed, univalve shells that are tenanted by the 

 Hermit-crab ; and there can be no doubt that its alliance 

 with the crustacean, though not essential to its wellbeing, 

 is at least the source of material advantage to it. I have 

 never found it in the situation in which Agassiz describes 

 his H.polyclina as frequently flourishing, on rocks in tide- 

 pools, where it sometimes covers, he says, "several square 

 feet with a rosy, velvet-like carpet," though it also occurs 

 on " the shells of Gasteropods, which serve as a retreat for 

 the Hermit-Crab." This zoophyte forms a whitish fleecy 

 covering on the shell of the mollusk, involving the greater 

 part of it when finely developed. The waving forest of 

 tall and graceful polypites generally reaches its greatest 

 height towards the mouth, round the edge of which are 

 set the curious snake-like appendages, either coiled up or 

 unrolled and cast out over the orifice like a fringe. Inter- 

 mingled with the perfect polypites, and commonly present 

 in immense numbers, are the rudimentary zooids, which 

 carry the generative sacs, attenuated by their work, and 

 looking as if weighed down by their burthen. Towards 

 the outskirts of the colony and along the growing edge of 



