CCELENTERATA. 55 



which becomes finally developed into a new and more or less 

 completely independent being. In fission, as in gemmation, 

 the new beings or zoo'ids may remain permanently in con- 

 nection with one another, when the process is a continuous 

 one, and a composite organism is produced, as in many corals. 

 Or, in other cases, the new zoo'ids produced by fission are de- 

 tached to lead an independent existence, as in some of the 

 Hydrozoa, the process thus becoming a discontinuous one. 



We are now able to understand what is meant, in strict 

 zoological language, by the term " individual," as applied to 

 animals. Zoologically speaking, an individual is defined as 

 "equal to the total result of the development of a single 

 ovum" In the higher animals there is no sort of difficulty 

 about this, for each ovum gives rise to no more than one 

 single animal, which cannot produce fresh beings in any other 

 way than by producing another ovum. In this case, there- 

 fore, each animal is an individual. In the lower animals, 

 however, the being produced by an ovum has often the power 

 of giving rise to fresh beings by a process of gemmation or 

 fission, and these beings may either remain attached to one 

 another so as to form a colony, or may become detached to 

 lead independent lives. In either case, the term " individual " 

 can only be properly applied to the whole assemblage of be- 

 ings produced in this way, however much they may differ 

 from one another in appearance, structure, or mode of life. 

 In these cases, therefore, the individual may be, firstly, a 

 single independent being as, for instance, an Amoeba, or an 

 Infusorian such as Paramoecium secondly, a colony or com- 

 posite organism composed of a number of more or less nearly 

 similar beings or zooids, produced by budding from a primi- 

 tive zooid as, for instance, a sponge, or such an Infusorian 

 as Epistylis ; and thirdly, an assemblage* of zo5ids produced 

 by budding or fission from a primitive being, but not neces- 

 sarily remaining connected with one another or exhibiting 

 any common features of likeness, as we shall see is the 

 case in many of the Hydrozoa. Lastly, cases may occur in 

 which the individual consists partly of similar zo5ids which 

 remain permanently connected with one another, and partly 

 of dissimilar zo5ids which are detached-to lead an independent 

 life, all alike being the result of the development of a single 

 ovum. 



Zooid (Gr. zodn, animal; eidos, form). The term "zooid" 

 is indifferently applied to all the more or less completely in- 

 dependent beings which are produced by budding, or by 



