62 INVEJiTEBKATE ANIMALS. 



common examples are the singular Hydractinice, which form brown 

 prickly crusts upon the empty shells of univalve Molluscs, invari- 

 ably selecting those shells which are inhabited by Hermit-crabs. 

 Other conunon types belong to Eudendrium. The genus Cordylo- 

 phora is found in fresh water ; and in CoryomorpJia we have a type 

 in which the hydrosoma consists of no more than a single polypite. 



As regards the generative process in the Corynida, it may be as 

 well to consider the general phenomena of reproduction as carried 

 on by all the Hydroid Zoophytes, the general characters of the pro- 

 cess being of a most remarkable nature. As has been already 

 explained, the " individual " in the case of the compound Hydrozoa 

 consists of an aggregation or colony of partially independent beings 

 or zooids, produced by gemmation or fission from a primordial 

 organism. This is the case in all composite animals, such as 

 Sponges, Sea-mats, Corals; and many others. In many of the com- 

 pound Hydrozoa, however, the case becomes still further compli- 

 cated. In many of these organisms, namely, the zooids differ very 

 much from one another both in structure and in function. One set 

 of zooids is entirely devoted to the duty of providing food for the 

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

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

 assemblage of them has been appropriately termed the "tropho- 

 some " (Allman), from the Greek treplio, I nouriah ; 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 

 re])roduction 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, off- 

 spring ; 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. 

 Tills difference in external appearance and in structure is sometimes 

 so great as to lead to a mast 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 endoderra (fig. 32), enclosing a cavity derived from the body- 

 cavity. In these buds the generative elements — ova and sperma- 

 tozoa — are developed. In other instances, the generative buds have 

 a more complicated structure. They consist now (fig. 29 m') of a 



