54 INVEETEBEATE ANIMALS. 



General Terminolooy of the Hydrozoa. 



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

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

 sary to glance briefly at the general features of reproduction as dis- 

 played in different sections of the animal kingdom. Reproduction 

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

 the perpetuation of the species ensured. This end may be attained 

 in vari(5us 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 amongst the higher animals, the 

 essential element of the process consists in the formation of two dis- 

 tinct structures, a germ-cell or ovum, and a sperm-cell or spermato- 

 zoid. By the union of these distinct repi-oductive elements, fresh 

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

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

 (male) ; but amongst the lower animals it is not uncommon for the 

 same individual to produce both of these elements, in which case 

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

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

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

 act of reproduction. The processes by which this can be effected in 

 different animals vary considerably, but they are all spoken of as 

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

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

 beings are produced by what is called " gemmation " or " fission.'' 



Gemmation (Lat. gemma, a bud) consists in the production 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 independent beings. The fresh beings thus produced by 

 budding are all known as zoiiids, and are not spoken of as distinct 

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

 zooids pi-oduced by budding remain permanently attached to one 

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

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

 residt of this is to produce a colony or com])osite structure, composed 

 of a number of similar and partially independent beings, all pro- 

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

 is seen very well in a great nmnber of the Hijdroma (fig. :i!»). When, 

 on the other hand, the zooids produced by budding become finally 

 detached from the jiarent organism, we have a case of what is called 

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

 come completely independent beings ; and they are often wholly 



