CCELENTERATA. 59 
In what is called “fission” (Lat. fizdo, 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 zodid. In fission 
the new beings are produced by a cleavage or division of a 
primitive zodid into two or more parts, each of which becomes 
finally developed into a new and more or less completely inde- 
pendent being. In fission, as in gemmation, the new beings or 
zoids 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 
zodids produced by fission are detached to lead an independent 
existence, as in some of the Hydrozoa, the process thus becom- 
ing 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 zudividual is defined as 
“egual 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, 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 beings preduced in this 
way, however much they may differ from one another in appear- 
ance, structure, or mode of life. In these cases, therefore, the 
individual may be, fszZy, a single independent being—as, for 
instance, an Ameba, or an Infusorian such as Paramecium ; 
secondly, a colony or composite organism composed of a num- 
ber of more or less nearly similar beings or zodids, produced 
by budding from a primitive zodid—as, for instance, a sponge, 
or such an Infusorian as Eistylis ; and thirdly, an assemblage 
of zodids produced by budding or fission from a primitive 
being, but not necessarily remaining connected with one an- 
other or exhibiting any common features of likeness, as we shall 
see is the case in many of the Hydrozoa. Lastly, cases may 
