HYDRA; ITS ACTIONS. 519 



oody of the Hydra consists of a simple bag, or sac, constituting 

 the stomach of the animal, and capable of varying its form and 

 dimensions to a very remarkable extent. In the largest species 

 it sometimes attains the length of an inch, when stretched out in 

 a cylindrical form ; whilst it will appear, in its contracted state, 

 as a small globe of scarcely perceptible dimensions. At the 

 upper end of this digestive sac is an opening, which may be 

 regarded as the mouth of the animal ; and round this are arranged 

 a certain number oftentacula, or long flexible arms, which diverge 

 from each other like the spokes of a wheel. If we look at the 

 mouth of the Hydra from above, therefore, we shall at once 

 perceive the claim of this animal to a place among the Radiated 

 Sub-Kingdom. The arms vary in number ; being usually from six 

 to ten. They vary also in dimensions ; not exceeding in the for- 

 mer species the length of the body ; whilst in the latter they often 

 extend themselves to as much as seven or eight inches, still being 

 able to contract themselves down to minute tubercles or knobs 

 projecting around the mouth. The animal, in its general aspect, 

 is thus seen to bear a close resemblance to the Cuttle-fish ;* and in 

 the peculiar organisation of its arms for the seizure of prey, this 

 resemblance, as we shall presently see, is extremely remarkable. 

 In fact, this little Polype may be regarded as one of those 

 sketches or fore-shadowings of higher forms, which we occasion- 

 ally meet with in the lowest groups. 



1 167. The arms are destitute of cilia ; and this is an import- 

 ant character, by which all the Polypes of the Hydra-form kind 

 may be at once distinguished from the Polyzoa ( 1070). They 

 are thickly set, however, with minute bristles ; besides which, 

 a number of little wart-like processes may be observed, from 

 whose summit sharp and firm spines are occasionally protruded, 

 by which a very firm hold is taken of whatever substance is 

 embraced by the animal. The mechanism by which these are 

 pushed out of their sheath is very curious. Each spine is 

 mounted upon the summit of a small vesicle, which is capable 



* It was this resemblance which caused Reaumur to give to the Hydrae the 

 name of Polypi ; the Polypi of the ancients being the animals now included in 

 the genus Octopus amongst the Cephalopoda. 



