54 INVERTEBRATE ANIMALS. 



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 eifected 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 considera- 

 tion, are those in which fresh beings are produced by what 

 is called " gemmation " or " fission." 



Gemmation (Lat. gemma, a bud) consists in the produc- 

 tion of a bud or buds, usually from the outside, but sometimes 

 from the inside, 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 zooids, and 

 are not spoken of as distinct animals for reasons which will be 

 immediately evident. When the zooids produced 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 result of this is 

 to produce a colony or composite structure, composed of a 

 number of similar and partially independent beings, aU pro- 

 duced by budding, but all remaining in organic connection. 

 This is seen very well in the sponges, in the compound Fora- 

 minifera, and in a great number of the Bydrozoa. When, on 

 the other hand, the zoOids produced by budding become finally 

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

 called " discontinuous " gemmation. In this case, the detached 

 zoOids become completely independent beings ; and they are 

 often wholly unlike the original zooid in structure and in 

 habits, so much so that they have in various cases been de- 

 scribed as altogether distinct animals. Discontinuous gem- 

 mation is very well seen in many of the Sydrozoa, and in 

 them the case is still further complicated by the coexistence 

 of discontinuous gemmation with the continuous form of the 

 process. Thus, it is not an uncommon thing among the 

 Hydrozoa to find a composite organism or colony produced 

 from a primordial zoSid by continuous gemmation, and hav- 

 ing at the same time the power of giving rise to detached and 

 completely independent beings by a process of discontinuous 

 gemmation. 



In what is called "fission" (Lat. findo, I cleave), exactly 

 the same results are attained as in gemmation, but in a 

 slightly different manner. In gemmation the new beings are 

 produced by means of buds thrown out by a primitive zo8id. 

 In fission the new beings are produced by a cleavage or 

 division of a primitive zoOid into two or more parts, each of 



