Royal Physical Society. 205 



fence or offence, like the motile spines and bird's-head pro- 

 cesses of the Polyzoa, or the pedicellariae of the Echinoder- 

 mata ; but I am unable to assign a reason for their peculiar 

 situation. They vary much in number and size in different 

 specimens of Hydractinia, but are rarely altogether absent. 



31. The Tentacular Polyp — (fig. 1 e, and fig. 13 b). — Not less 

 surprising than the polyps last described are these, the great 

 tentacles of the polypary. When a specimen of Hydractinia 

 is allowed to rest suspended in a glass jar of water, these organs 

 are extended to a distance of three, four, or even five times 

 the length of the alimentary polyps, and hang down, loosely 

 floating in the water, like the thread-like tentacles of the long- 

 armed hydra. They are found on the outskirts of the poly- 

 pary, and on each side of the long diameter of the mouth of 

 the shell, so that they must, in their natural condition, reach to 

 the ground, and enable the zoophyte to seize food scattered 

 there by the feeding crab. The tips are covered with a dense 

 pavement of the larger thread-cells ; and a few of the same 

 bodies are thinly scattered along their whole length. As far 

 as I know, no organ analogous to them exists in any other 

 hydroid zoophyte. They have not been hitherto described. 



32. In our consideration of the subject of this communica- 

 tion our attention is arrested by the multitude of objects 

 grouped together to constitute a single animal, their variety in 

 form, and the sympathy which subsists between the different 

 parts. The singular spinous skeleton; the expanded mem- 

 brane of the polypary, with its beautiful internal network of 

 tubes and delicate peripheric prolongations ; the alimentary 

 polyps, some white and filiform, others thick, fleshy, crimson, 

 or yellow sacs, obligingly everted, to expose their interior to 

 our microscopic eye ; the reproductive polyps, with their richly- 

 coloured generative sacs ; the sessile generative organs of the 

 polypary ; the ophidian polyps, coiled in neat spirals when at 

 rest, but starting into furious action, like a row of well-drilled 

 soldiers, when injury is inflicted on the body to which they are 

 attached • and lastly, the tentacular polyps, floating in the 

 water like long -and slender threads of gossamer, or dragging up 

 heavy loads of food for the common good ; these, together with 

 the intimate relation and sympathy subsisting between the 



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