68 INVERTEBRATE ANIMALS. - 
of the process being of a most remarkable nature. As has 
been already explained, the individual in the case of the com- 
pound //ydrozoa consists of an aggregation or colony of par- 
tially independent beings or zodids, produced by gemmation or 
fission from a primordial organism. This is the case in all com- 
posite animals, such as sponges, sea-mats, corals, and many 
others. In many of the compound Aydrozoa, however, the case 
becomes still further complicated. In many of these organisms, 
namely, the zodids differ very much from one another both in 
structure and in function. One set of zodids is entirely devoted 
to the duty of providing food for the colony, and in these no 
reproductive organs are ever developed. These nutritive 
zodids are all like each other in form, and the whole assem- 
blage of them has been appropriately termed the “trophosome” 
(Allman), from the Greek ¢vefho, I nourish; and soma, body. 
The colony or trophosome thus formed by the nutritive zodids 
can go on increasing by the production of fresh zodids for an 
almost indefinite period ; but in all cases there ultimately comes 
a time when it becomes necessary to produce the essential ele- 
ments of reproduction in order to secure the perpetuation of 
the species. The nutritive zodids, as just stated, cannot pro- 
duce the ova and sperm-cells, being destitute of reproductive 
organs, and the colony is therefore compelled to produce a 
second set of buds, which have the power of producing the 
essential elements of reproduction. These buds are collectively 
called the ‘“‘gonosome” (Gr. gozos, offspring ; and soma, body). 
The generative buds have the further peculiarity that not only 
can they produce the generative elements, but they are alto- 
gether unlike the nutritive zodids in appearance. This differ- 
ence in external appearance and in structure is sometimes so 
great as to lead to a most remarkable series of phenomena. In 
the simplest form in which these generative buds or “gono- 
phores” appear, they have the form of mere protuberances of 
the ectoderm and endoderm (fig. 19, a), enclosing a cavity 
derived from the body-cavity. In these buds the generative 
elements—ova and spermatozoa—are developed (fig. 18, 4). In 
other instances, the generative buds have a more complicated 
structure. They consist now (fig. 19, c) of a bell-shaped disc, 
which is attached by its base to the parent organism, and has 
its cavity turned outwards (see also fig. 18, ¢c). From the 
roof of this disc there is suspended a kind of handle, which 
