CCELENTERATA. 55 



unlike the original zooid in structure and in habits, so much so 

 that they have in various cases been described as altogether dis- 

 tinct animals. Discontinuous gemmation is very well seen in many . 

 of the Hydrozoa, 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 amongst 

 the Hydrozoa to find a composite organism or colony produced from 

 a primordial zooid by continuous gemmation, and having at the 

 same time the power of giving rise to detached and completely inde- 

 pendent 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 zooid. In fission the new beings 

 are produced by a cleavage or division of a primitive zooid into two 

 or more parts, each of which becomes finally developbd into a new 

 and more or less completely independent being. In fission, as in 

 gemmation, the new beings or zooids may remain permanently in 

 connection 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 zooids produced by fission are detached 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. Zoo- 

 logically 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, 

 therefore, each animal is an individual. In the lower animals, how- 

 ever, 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 beings 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 Infusorial! such as Paramcecium ; secondly, a colony or composite 

 organism composed of a number of more or less nearly similar 

 beings or zooids, produced by budding from a primitive zooid — as, 

 for instance, is seen in many of the Infusoria {e.g., Epistylis); and 



