CCELENTERATA : HYDROZOA. 



CLASS I. HYDROZOA. 



The Hydrozoa are defined as Ccelenterata in which the walls 

 of the digestive sac are not separated from that of the general 

 body-cavity, the two coinciding with one another; the reproductive 

 organs are in the form of external processes of the body-wall. 

 (Fig. 37, B.) 



Fig- 37- A, The common Hydra (Hydra vulgaris), carrying young Hydrce which it 

 has produced by budding, considerably magnified (after Hincks). B, Diagrammatic 

 section of the Hydra, showing the mouth surrounded by the tentacles, and the disc 

 of attachment ; the dark and light lines indicate the two layers of the integument, 

 and on one side of the body is shown a single large egg. 



It follows from the above, that, since there is but a single 

 internal cavity, the body of a Hydrozoon on transverse section 

 appears as a single tube, the walls of which are formed by the 

 limits of the combined digestive and somatic cavity. 



The Hydrozoa are all aquatic, and the great majority are 

 marine. The class includes both simple and composite organ- 

 isms, the most familiar examples being the common Fresh-water 

 Polype (Hydra), the Sea -firs (Sertularidd), the Jelly-fishes 

 (Medusa), and the Portuguese man-of-war (Physalia). 



Owing to the great difficulty which is ordinarily experienced by the stu- 

 dent in mastering the details of this class of animals, it has been thought 

 advisable to introduce here a short explanation of some of the technical 

 terms which are in more general use in describing these organisms. 



