CCELENTEKATA. 53 



nozoa, on the other hand, there is a distinct digestive cavity, 

 but this opens directly into the general body-cavity, so that 

 the two form distinct but freely-communicating divisions of 

 the same chamber. 



Class I. — Htdeozoa. 



The Mydrozoa are defined as Codenterata in which the 

 walls of the digestive sac are not separated from those of the 

 general cavity of the body, the two coinciding with one an- 

 other. T/ie reproductive organs are external, in the form of 

 outward processes of the body-wall (Fig. 13, a, b). 



The Hydrozoa are all aquatic in their habits, and, with 

 the exception of two genera, aU are inhabitants of salt water. 

 The class includes both simple and composite organisms, of 

 which the most familiar are the sea-firs and their allies (Hy- 

 droid zoophytes), the fresh-water polype or Hydra, the sea- 

 jellies {Medusae), and the Portuguese man-of-war (Physalid). 

 Owing to the extremely complicated nature of many of the 

 Hydrozoa, it appears advisable to preface their description 

 by an explanation of some of the more important terms which 

 are employed in connection with various members of the 

 class. 



Gexeeal Teemixology of the Htdeozoa. 



Individual. — In order to understand fully the meaning 

 which is attached to the term " individual " in zoological lan- 

 guage, it is necessary to glance briefly at the general features 

 of reproduction as displayed in diflferent sections of the ani- 

 mal kingdom. Reproduction is the process by means of which 

 new individuals are produced and the perpetuation of the 

 species insured. This end may be attained in various ways, 

 but these all come under the two heads of " sexual " and 

 " non-sexual " reproduction. In sexual reproduction, by which 

 alone can fresh beings be produced among the higher ani- 

 mals, the essential element of the process consists in the 

 formation of two distinct structures, a germ-cell or ovum, and 

 a sperm-cell or spermatozoid. By the union of these distinct 

 reproductive elements fresh beings can be produced. As a 

 general rule, the germ-cell is produced by one individual 

 (female), and the sperm-cell by another (male) ; but among 

 the lower animals it is not uncommon for the same individual 

 to produce both of these elements, in which case the indi- 

 vidual is said to be "hermaphrodite." Among the lower 

 animals, however, fresh beings may be produced without the 



