482 



ORDER I. HYDROIDA. 



1043. The Hydra, to which reference has been made as the 

 type of one of the divisions of the Polypifera, is a minute animal, 

 common in stagnant pools of water, 

 where numbers are often found 

 clustering upon aquatic plants, or 

 other floating bodies. Although 

 most of the species prefer fresh 

 water, some are occasionally found 

 at sea. These curious little crea- 

 tures possess an organisation which 

 appears very simple, and so it has 

 long been considered; but recent 

 improvements in the power of the 

 microscope have enabled its struc- 

 ture to be more fully analysed, and 

 have revealed (in this as in every 

 other instance) details that were 

 previously unsuspected. The Hydra 

 viridis, or Green Polype, and the 



Hydra fusca, or Brown Polype, are the two best known species ; 

 and to these our description will chiefly apply. The body 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 of tentacula^ 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 



FIG. 615. HYDRA. 



