458 



ZOOPHYTES. 



FiO. 220. 



end into a narrow base, which is furnished with a suctorial disk; 

 and the Hydra usually attaches itself by this, whilst it allows its 



tendril-like tentacula to float 

 freely in the water, like so many 

 fishing-lines. The wall of the 

 body is composed of cells, im- 

 bedded in a kind of sarcode; 

 and it consists of two principal 

 layers, an outer and more com- 

 pact, of which the cells form a 

 tolerably even surface, and an 

 inner that lines the stomach, 

 into the cavity of which some of 

 the cells project. Between these 

 layers, there is a space chiefly 

 occupied by " sarcode," having 

 many vacuoles or lacunae (which 

 often seem to communicate with 

 one another) excavated in its 

 substance. The arms are made 

 up of the same materials as the 

 body ; but their surface is beset 

 with little wart-like prominences, 

 which, when carefully examined, 

 are found to be composed oi 

 clusters of small "thread-cells," 

 having a single large cell with 

 a long spiculum in the centre of 

 each. The structure of these 

 thread-cells or "urticating or- 

 gans" will be described hereafter 

 (§ 310) ; at present it Avill be 

 enough to point out, that this ap- 

 paratus, repeated many times on each tentacle, is doubtless in- 

 tended to give to the organ a great prehensile power ; the minute 

 fllaments forming a rough surface, adapted to prevent the object 

 from readily slipping out of the grasp of the arm, whilst the 

 central spiculum is projected into its substance, and probably 

 conveys into it a poisonous fluid secreted by a vesicle at the base 

 of the dart. The latter inference is founded upon the oft-re- 

 peated observation, that if the living prey seized by the tentacles 

 have a body destitute of hard integument, as is the case with 

 the minute aquatic Worms which constitute a large part of its 

 aliment, this speedily dies, even if, instead of being swallowed, 

 it escapes from their grasp ; on the other hand, minute Euto- 

 mostracous Crustacea, Insects, and other animals with hard en- 

 velopes, may escape without injury, even after having been de- 

 tained for some time in the polype's embrace. The contractility 

 of the tentacula (the interior of which is traversed by a canal 



Hydra fusca, with a young bud at &, and 

 a more advanced bud at c. 



