7° INVERTEBRATE ANIMALS, 
identical structurally with the smaller forms of the so-called 
sea-jellies or Medus@; and it is now known that most if not 
all of these Meduse, though originally described as distinct 
beings, are really nothing more than the free generative buds 
of the fixed AHydrozoa. We have here, then, an instance of 
what has been not quite appropriately called “alternation of 
generations.” 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 off 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 ele- 
ments. When once 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 “ medusi- 
form” 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 gonophore, in which the ova and sperm-cells are developed. 
The production of this gonophore from the parent organism 
(trophosome) 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 ectederm 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 disc. The reproductive elements are 
developed either in the walls of these canals or between the 
