158 THE MECHANICAL FUNCTIONS. 



water by their own spontaneous movements, con- 

 sisting either in the waving up and down of the 

 lateral branches, or in the simtdtaneous impulses 

 of the tentacula of all the polypes. Cuvier even 

 represents the polypes of the pennatula as having 

 the power of keeping time, while they are waving 

 the mass through the water, as if they were all 

 actuated by a single undivided volition. But Dr. 

 Grant concludes from his observations that penna- 

 tulae are not in reality possessed of any such loco- 

 motive faculty ; but that they are carried to and 

 fro in the ocean, like the gulf weed, without the 

 slightest voluntary power of directing their course. 

 Whatever may be the result of the combined 

 movements of the tentacula, the arms are certainly 

 incapable of those inflexions which have been sup- 

 posed to supply the means of progressive motion. 



It is only when the contractile flesh of the 

 polypus is released from the restraint which the 

 solid axis imposes on its movements, that the animal 

 becomes capable of any distinct power of locomo- 

 tion. Such is the condition of the animals belong- 

 ing to the genus Hydra, of which the Hydra viridis, 

 or fresh water polype (Fig. 59, p. 147), 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 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 



