518 



HYDROZOA ; HYDRA. 



visable to treat the Hydroid Polypes and Medusae provisionally 

 as forming distinct Orders ; the number of such groups included 

 in the class will then be four, namely 



I . HYDROIDA, or the Hydroid Polypes, including all the at- 

 tached polypiform species, whether simple or compound. 



II. DISCOPHORA, Medusce or Jelly-fish, in which the body con- 

 sists of a convex disc or umbrella, from the centre of the lower 

 surface of which depends a peduncle containing the stomach, 

 and terminated by the mouth. 



III. SIPHONOPHORA, which appear to consist of free-swim- 

 ming colonies of Polypes, analogous to the simple Hydroida. 



IV. CTENOPHORA, simple animals, the motions of which 

 through the water are effected by the agency of bands of cilia 

 which run along their surface. 



ORDER I. HYDROIDA. 



1 166. The Hydrate creature to which the name of Polype was 

 first applied in modern times, is a 

 minute animal, common in stag- 

 nant pools of water, where num- 

 bers are often found clustering 

 upon aquatic plants, or other float- 

 ing bodies. These curious little 

 creatures 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 

 structure 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 



FIG. 72X HYDHA. 



