DIVISIONS OF THE HYDROZOA. 63 



is entirely devoted to the duty of providing food for the col- 

 ony, and in these no reproductive organs are ever developed. 

 These nutritive zooids are all like each other in form, and the 

 whole assemblage of them has been appropriately termed the 

 "trophosome" (Allman), from the Greek trepho,! nourish; 

 and soma, body. The colony or trophosome thus formed by 

 the nutritive zooids can go on increasing by the production 

 of fresh zooids for an almost indefinite period ; but in all cases 

 there ultimately comes a time when it becomes necessary to 

 produce the essential elements of reproduction in order to 

 secure the perpetuation of the species. The nutritive zooids, 

 as just stated, cannot produce 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. 

 gonos, offspring; and soma, body). The generative buds 

 have the further peculiarity that not only can they produce 

 the generative elements, but they are altogether unlike the 

 nutritive zooids in appearance. This difference 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 " gonophores " appear, 

 they have the form of mere protuberances of the ectoderm 

 and endoderm (Fig. 16, a), enclosing a cavity derived from 

 the body-cavity. In these buds the generative elements 

 ova and spermatozoa are developed (Fig. 15, b). In other 

 instances, the generative buds have a more complicated struct- 



FIG. 16. Generative buds or gonophores of the Hydrozoa diagrammatically represented. 

 a Simple gonophore, consisting merely of a protuberance of the ectoderm and endo- 

 derm ; c Gonophore which has the structure of a Medusa (medusoid), but is not de- 

 tached ; d Free medusiform gonophore. 



ure. They consist now (Fig. 16, c) of a bell-shaped disk, 

 which is attached by its base to the parent organism, and 

 has its cavity turned outward (see also Fig. 15, c). From 



