HYDRA; ITS ACTIONS. 483 



to ten. They vary also in dimension ; 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-shadoicings of higher forms, which we occasionally 

 meet with in the lowest groups. 



1044. 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 those of a higher group 

 ( 1095). 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 occa- 

 sionally protruded, by which a very firm hold is taken of what- 

 ever 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 of expanding and contracting within an 

 envelope, that embraces the whole apparatus. When at rest, 

 this vesicle lies at the bottom of the including sac, and the spine 

 is drawn entirely within it ; but, when the animal lays hold of 

 any object with one or more of its arms, the vesicle is distended 

 by some unseen means, and protrudes the spine which is seated 

 upon it. 



1045. When in search of prey, the Hydra permits its arms to 

 float loosely through the water. It is rather curious that so 

 inactive a creature as this should principally depend for its food 

 on the minute Crustacea and aquatic Worms, whose rapid move- 

 ments would seem to place them beyond its reach. By lying in 

 wait, however, with its arms thus disposed, the Polype soon 



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

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

 genus Octopus of the Cephalopoda. 



