34 MANUAL OF ZOOLOGY. 



capable of performing all the functions of life. In this case, 

 therefore, each one of the innumerable buds becomes an inde- 

 pendent being, similar to, though not detached from, the organ- 

 ism which gave it birth. This is an instance of what is called 

 " continuous gemmation." 



In other cases as in the common fresh-water polype or 

 Hydra the buds which are thrown out by the primitive or- 

 ganism become developed into creatures exactly resembling 

 the parent, but, instead of remaining permanently attached, 

 and thus giving rise to a compound organism, they are de- 

 tached to lead an entirely independent existence. This is a 

 simple instance of what is termed " discontinuous gemmation." 



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 compo- 

 site 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 

 language, an individual is defined as " equal to the totalresult 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 important, 

 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 sepa- 

 rate 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 a zoo id. Here the zooids are not perma- 

 nently 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 "in- 

 dividual " 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 



