DIVISIONS OF THE HYDROZOA. 65 



We have a compound fixed animal, in many respects comparable to 

 a plant, producing a special series of buds which are devoted to the 

 process of reproduction. These buds are cast otf as independent 

 beings to lead an independent life, and they are furnished with the 

 necessary organs to preserve their existence till they are able to 

 mature the reproductive elements. When once able to consummate 

 this, they die ; but the young to which they give oiigin are wholly 

 unlike themselves. The young, namely, instead of being free- 

 swimming " medusiform " 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 alto- 

 gether good one, and does not quite express the facts of the case. 

 There is not any alternation of generations, but there is an alter- 

 nation of generation with gemmation or budding. The only truo 

 generative act takes place in the reproductive zooid or gonophoi e, 

 in which the ova and sperm-cells are developed. The production of 

 this gonophore from the parent organism (trophosome) is a process, 

 not of genei'ation, but of gemmation or budding. The whole pro- 

 cess, therefore, is, properly speaking, not an "alternation of gener- 

 ations," 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 gono- 

 phores, 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 dis- 

 tinguished : 1. Simple closed sacs or protuberances (" sporosacs ") 

 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 manubrium, 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 disc. The reproductive elements are 

 developed either in the walls of these canals or between the ecto- 

 derm and endoderm of the manubrium. From the resemblance of 

 these buds in anatomical structure to the so-called Sea-jellies or 

 Medusce, they are usually spoken of as "medusiform gonophores," 

 or simply as " medusoids." In this form, however, though highly 

 organised, the buds never become detached from the parent colony. 

 3. Buds which become developed into bell - shaped medusiform 

 bodies exactly similar in structure to the last, but detached to lead 

 an independent existence. These free-swimming medusiform gono- 

 phores are anatomically indistinguishable from ordinary Medusa;; 

 and it is now known that most of the so - called " naked - eyed " 

 Medusm, are really the detached generative buds of other orders of 



