DIVISIONS OF THE HYDKOZOA. 65 



able to consummate this, they die ; but the young to which 

 they give origin are wholly unlike themselves. The young, 

 namely, instead of being free-swimming " medusifomi " beings, 

 become developed into the fixed, plant-like colony from which 

 the generative buds were originally produced. The term 

 " alternation of generations " is not an altogether good one, 

 and does not quite express the facts of the case. There is 

 not any alternation of generations, but there is an alternation 

 of generation with gemmation or budding. The only true 

 generative act takes place in the reproductive zooid or gono- 

 phore, in which the ova and sperm-cells are developed. The 

 production of this gonophore from the parent organism (tro- 

 phosome) is a process, not of generation, but of gemmation 

 or budding. The whole process, therefore, is, properly speak- 

 ing, not an " alternation of generations," but an alternation 

 of generation with gemmation. 



To recapitulate, then — the process of reproduction in the 

 Hydroid zoophytes is carried on by means of reproductive 

 buds or gonophores, which are produced at special seasons, 

 and in which the reproductive elements are developed. These 

 generative buds differ a good deal in their character, but three 

 chief kinds may be distinguished : 1. Simple closed sacs or 

 protuberances formed out of both ectoderm and endoderm, 

 and having the special elements of generation developed in 

 their interior. 2. Bell-shaped buds, attached to the parent 

 colony by their bases, and having a central process or manu- 

 brium, which is furnished with a mouth and central cavity, 

 from which there is given off a system of canals to ramify in 

 the substance of the disk. The reproductive elements are 

 developed either in the walls of these canals or between the 

 ectoderm and endoderm of the manubrium. From the resem- 

 blance of these buds in anatomical structure to the so-called 

 sea-jellies or Medusce, they are usually spoken of as " medusi- 

 form gonophores," or simply as " medusoids." In this form, 

 however, though highly organized, the buds never become de- 

 tached from the parent colony. 3. Buds which become de- 

 veloped into bell-shaped medusiform bodies exactly similar in 

 structure to the last, but detached to lead an independent ex- 

 istence. These free-swimming medusiform gonophores are 

 anatomically indistinguishable from ordinary Medusae ; and it 

 is now known that most, if not all, of the so-called " naked- 

 eyed " Medusce, are really the detached generative buds of 

 other orders of Sydrozoa. The special elements of reproduc- 

 tion are developed in these detached buds, but the resulting 



