28 MANUAL OF ZOOLOGY. 



The method and results of fission may be regarded as essen- 

 tially the same as in the case of gemmation. The products 

 of the division of the body of the primitive organism may 

 either remain undetached, when they will give rise to a com- 

 posite structure (as in many corals), or they may be thrown 

 off and live an independent existence (as in some of the 

 Hydrozoa). 



We are now in a position to understand what is meant, 

 strictly speaking, by the term " individual." In zoological lan- 

 guage, an individual is defined as " equal to the total result of 

 the development of a single ovum" Amongst the higher animals 

 there is no difficulty about this, for each ovum gives rise to no 

 more than one single being, which is incapable of repeating it- 

 self in any other way than by the production of another ovum ; 

 so that an individual is a single animal. It is most import- 

 ant, however, to comprehend that this is not necessarily or 

 always the case. In such an organism as the sea-mat, the 

 ovum gives rise to a primitive polypide, which repeats itself by 

 a process of continuous gemmation until an entire colony is 

 produced, each member of which is independent of its fellows, 

 and is capable of producing ova. In such a case, therefore, 

 the term " individual " must be applied to the entire colony, 

 since this is the result of the development of a single ovum. 

 The separate beings which compose the colony are technically 

 called " zooids." In like manner the Hydra, which produces 

 fresh and independent Hydrae by discontinuous gemmation, is 

 not an " individual," but is a zooid. Here the zooids are not 

 permanently united to one another, and the "individual " Hydra 

 consists really of the primitive Hydra, plus all the detached 

 Hydrae to which it gave rise. In this case, therefore, the " indi- 

 vidual " is composed of a number of disconnected and wholly 

 independent beings, all of which are the result of the develop- 

 ment of a single ovum. It is to be remembered that both the 

 parent zooid and the " produced zooids " are capable of giving 

 rise to fresh Hydrae by a true generative process. It must 

 also be borne in mind that this production of fresh zooids by a 

 process of gemmation is not so essentially different to the true 

 sexual process of reproduction as might at first sight appear, 

 since the ovum itself may be regarded merely as a highly spe- 

 cialised bud. In the Hydra, in fact, where the ovum is pro- 

 duced as an external process of the wall of the body, this like- 

 ness is extremely striking. The ovarian bud, however, differs 

 from the true gemmae or buds in its inability to develop itself 

 into an independent organism, unless previously brought into 

 ' contact with another special generative element. The only 



