62 INVEliTEBKATE ANIMALS. 



common examjiles are the singular Hi/dracUnue, which form brown 

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

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

 Other common types belong to Biidendrium. The genus Cordylo- 

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

 in which the hyilrosoma consists of no nicjre than a single polypite. 



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

 well to consider the general ]ihenomena of reproduction as carried 

 on T)y all the Hydroid Zoojjhytes, the general characters of the ]jro- 

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

 explained, the " individual " in the case of the comjjound Hijdmzoa 

 consists of an aggregation or colony of partially independent beings 

 or zooids, ]iroduced by gennnation or fission from a primordial 

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

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

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

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

 much from one another both in structure and in i'unctiou. One set 

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

 colony, and in these no reproductive organs ai-e ever develo])ed. 

 These nutritive zooids are all like each other in foi-m, and the whole 

 assendjlage of them has been ajipropriately termed the "tropho- 

 some " (Allman), from the (!ireek trepho, I nourish ; and soma, body. 

 The colony or ti'ophosome 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 

 reiiroduction in order to secure the perpetuation of the species. 

 The nutiitive zooids, as just stated, cannot produce the ova and 

 sperm-cells, lieing destitute of re])roductive 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" {Qiv. gonos, off- 

 spring ; and soma, body). The generative buds have the further 

 peculiarity that not only can they ))roduce the generative elements, 

 but they are altogether unlike the nutiitive zooids in ajipearance. 

 This dilference in e.\ternal appearance and in structure is sometimes 

 so great as to lead to a most remarkable series of i)henoniena. In 

 the simplest form in which these generative buds or " gonophores " 

 apjie.'ir, they have tlie fnriii (if mere ] irotuberances of the ectoderm 

 and endoilerm (fig. ?>■!), enclosing a, cavity derived from the body- 

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

 t'«o,i- ;ire ileve]oi>ed. In other in.stances, the generative biids have 

 a mine cunjplicatcd .structure. They consi.st now (fig. '29 ?«') of a 



