132 THE MECHANICAL FUNCTIONS. 



animals belonging to the genus Hydra, of which the Hydra 

 viridis, or fresh water polype, (Fig. 59, p. 122) may be taken 

 as the type. This singular animal presents us with perhaps 

 the simplest kind of structure that exists in the animal king- 

 dom. It would almost seem as if Nature had formed it with 

 the design of exhibiting to us the resources of vitality in 

 carrying on the functions of animal life without the aid of 

 the complicated apparatus which she has bestowed upon the 

 higher orders of the creation. The Hydra consists merely of 

 a fleshy tube, open at both ends, one of which, being more 

 dilated, may be regarded as the head, and has for a mouth 

 the aperture of the tube, which is furnished at its margin 

 with a single row of tentacula. It thus corresponds to the 

 general definition of a polypus, and exemplifies its most 

 simple form. 



The whole body may, on the one hand, be considerably 

 elongated, and on the other, so much retracted, as to appear 

 a mere globule; and these movements are the efiect of a vo- 

 luntary power in the animal directed to specific ends. The 

 number of tentacula varies from six to twelve; they are slen- 

 der tubular filaments, capable of being extended to a great 

 length, and of being bent in all directions. In this way, 

 they can quickly surround and grasp any small object which 

 they may happen to touch; and whenever irritated they in- 

 stantly retract, so as hardly to be visible without the aid of 

 a magnifier. Each tentaculum may be moved independent- 

 ly of the rest, at the pleasure of the animal. The remainder 

 of the body tapers gradually from the head to the other ex- 

 tremity, becoming very slender, and having at its termina- 

 tion a flat surface, which has been termed the foot; for al- 

 though every portion of the surface has the power of ad- 

 hering to the bodies to which it is applied, it is principally 

 by this extremity that the animal chooses to attach itself to 

 the sides or bottom of the vessel in which it is kept. No 

 trace of the existence of cilia can be met with on any part 

 of the surface of these animals. 



It is to Mr. Trembley of Geneva that we are indebted for 

 the discovery of this singular animal, the examination of 



