176 THE MECHANICAL FUNCTIONS. 



the solid axis imposes upon its movements, that 

 the animal becomes capable of any distinct 

 power of locomotion. Such is the condition 

 of the animals belonging to the genus Hydra, 

 of which the Hydra viridis, or fresh water 

 polype, (Fig. 59, p. 162) may be taken as the 

 type. This singular animal presents us with 

 perhaps the simplest kind of structure that exists 

 in the animal kingdom. It would almost seem 

 as if Nature had formed it with the design of 

 exhibiting to us the resources of vitality in carry- 

 ing 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 ten- 

 tacula. It thus corresponds to the general defi- 

 nition 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 effect of a voluntary 

 power in the animal directed to specific ends. 

 The number of tentacula varies from six to 

 twelve ; they are slender tubular filaments, ca- 

 pable of being extended to a great length, and 



