54 INVEKTEBRATE ANIMALS. 



fiENERAL Terminology op the Hydrozga. 



Individual. — In order to iimlerstand fully the meaning which is 

 attiiched to the term " individual " in zoological language, it is neces- 

 saiy to glance briefly at the general features of repi'oduction as dis- 

 played in different sections of tlie animal kingdom. Reproduction 

 is the process by means of which new individuals are produced and 

 the per]ietuation of the species ensured. This end may he attained 

 in various ways, but these all come under the two heads of " sexual " 

 and " non-sexual " rejiroduction. In sexital reproduction, by which 

 alone can fresh beings be jiroduced amongst the higher animals, the 

 essential element of the ]>rocess consists in the formation of two dis- 

 tinct structures, a gei'm-ccU or ovum, and a sperm-cell or spermato- 

 zoid. By the union of these distinct reproductive elements, fresh 

 beings can be jiroduced. As a general rule, the germ-cell is pro- 

 duced by one individual (female), and the sperm-cell by another 

 (ni.ile) ; but among.st the lower animals it is not unconmiou i»v the 

 same individual to jiroduce both of tliese elements, in which case 

 the individual is said to be "hermaphrodite." Amongst the lower 

 animals, however, fi'esh beings may be pi'oduced without the contact 

 of a sjierm-cell and an ovum — that is to say, without any genuine 

 act of rei>roducti(jn. The jjrocesses by which this can be effected in 

 ditf'erejit animals vary considerabl}', but they are all spoken of as 

 foinis of " non-sexual " reproduction. The only varieties, however, 

 of the process which require consideration, are those in wdiich fresh 

 beings are jiroduced by what is called "gemmation" or "tissiou." 



(jemmation (Lat. gumma, a bud) consists in the ]iroduction of a 

 bud or buds, usually from the outside, but sometimes from the in- 

 side of an animal ; which buds become developed into more or less 

 completely inde]iendent beings. The fresh beings thus produced by 

 budding are all known as zviiids, and are not spoken of as di.stiuct 

 animals, for reasons which will be immediately evident. When the 

 zooids Jiroduced by budding remain jjermaneutly attached to one 

 another and to the parent organism which jjroduced them, the case 

 is said to be one of "continuous" genmiation, and the ultimate 

 residt of this is to jiroduce a colony or comjKjsite structure, comjiosed 

 of a nundier of similar and partially inde|iendent beings, all pro- 

 duced Ijy budding, but all remaining in organic connection. This 

 is sei-'n very well in a great number of the I/i/dro:oa (Hg. 2!)). When, 

 on the otlier hand, the zooids jiroduced by budding become finally 

 di'tached from the jiarent organism, we have a. case of what is called 

 "discontiiuious " gemmation. In this case, the detached zooids be- 

 come ciiuijiletfcly indejiendent beings ; and they are often wholly 



