102 ELEMENTS OF BIOLOGY. 



self-division or cleavage, which may take place either trans- 

 versely or longitudinally. In either case a groove is formed 

 on the exterior surface, which gradually deepens, till the 

 original organism is split up into two similar and indepen- 

 dent Paramcecia. It would appear, however, that the initia- 

 tive in the process of fission is taken by the reproductive 

 organs in the interior of the body, which first divide into 

 two similar halves. 



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

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

 language, an individual is defined as " equal to the total 

 residt of the developme?it of a sifigle ovum." Amongst the 

 higher anijpals there is no difficulty about this, for each 

 ovum gives rise to no more than one single being, which is 

 incapable of repeating itself in any other way than by the 

 production of another ovum ; so that an individual is a 

 single animal. It is most important, however, to compre- 

 hend that this is not necessarily or always the case. In 

 such an organism as the sea-mat (fig. 27), 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 techni- 

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

 produces fresh and independent Hydrse 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 "individual" is composed of a 

 number of disconnected and wholly independent beings, all 

 of which are the result of the development of a single ovum. 

 It is to be remembered that both the parent zooid and the 



