194 CCELENTERATA. 



an outer and inner membrane, known respectively as the " ectoderm " 

 and " endoderm," and no hard external layer is produced. The 

 mouth opens into a cylindrical body-cavity or " ccelenteric space," 

 which runs the whole length of the body, and is completely undi- 

 vided, no gullet being developed. There are, therefore, no radiating 

 membranous partitions corresponding with the " mesenteries " of the 

 Actinozoa. The animal can produce buds, which become devel- 

 oped into new " polypites," but these do not remain attached to the 

 parent polypite, and the organism does not, therefore, become com- 

 posite. Moreover, at certain seasons, the animal develops true 

 reproductive organs, which have the form of external buds. These 

 discharge the reproductive elements by rupture of their walls, but are 

 not themselves separated from the organism. 



The ovum in all the Hydrozoa gives rise to a " polypite " similar in 

 structure to the entire organism in Hydra; but the primordial poly- 

 pite is often endowed with the power of throwing out buds which 

 remain, some or all of them, permanently united with the original 

 " zooid." In this way is produced a composite organism (fig. 76, b, 

 and fig. 77), consisting of a greater or smaller number of zooids or 

 "polypites," united by a common fleshy stem or " ccenosarc." Very 

 generally, the zooids become differentiated into two sets, which differ 

 in structure and in the part they play in the life of the colony. In 

 the one series the zooids constitute the ordinary nutritive "polypites " 

 of the colony (fig. 77,/), and are concerned with supplying food to 

 the organism. In the other series the zooids — now spoken of by the 

 general name of the " gonophores " — have a reproductive function, 

 and are concerned with the production of the generative elements. 

 These generative zooids or " gonophores " may remain permanently 

 attached to the parent colony (fig. 7 6, b), or they may become much 

 modified and may become detached to lead a free existence (fig. 77). 

 In this latter case, the detached generative zooids usually present 

 themselves in the form of "Jelly-fishes" or " Medusoids." 



The colonies of the composite Hydrozoa are sometimes free, the 

 proximal end of the ccenosarc not being adapted for fixation (as in 

 the Siphonophora and the Graptolitoidea proper). In other cases 

 the colony is attached to some foreign body by a modified proximal 

 extremity or "hydrorhiza" (fig. 76, rK). 



In a number of the composite Hydrozoa no hard structures in the 

 shape of an external skeleton are developed. In many cases, how- 

 ever, the ectoderm secretes a firm horny outer layer or " periderm " 

 (fig. 76, pe), which may cover the ccenosarc only (as in Bougain- 

 vilka, fig. 77), or may be extended into little cups or " hydrothecae " 

 within which the individual polypites are contained. When " hydro- 

 thecae " are present, the body of the polypite is contained within 

 the cup, and the distal extremity, with the mouth and tentacles, 



