250 M. C. Mereschkowsky on the Hydroida. 
forms and in general all the morphological facts presented by 
the Hydroids, and which I think may contribute somewhat to 
the better comprehension of the idea of the Hydroid, so to 
speak, and to concentrate all the differences presented by this 
group in a single representation. 
Every one at present regards a hydranth, with its tentacles, 
as a single individual, furnished with organs radially ar- 
ranged. For my part, I believe we must regard such an 
organism, not as an individual, but as a colony composed of 
two kinds of individuals—the one kind appropriated to the 
function of seizing food, with the gastral cavity but slightly 
developed, without a mouth, very flexible and thin (tentacles) ; 
the other destined exclusively to nourish the whole colony, 
furnished with a mouth, and with a large cavity in the body 
which is but slightly flexible (the actual body of the hydranth). 
We should thus have a polymorphic colony after the fashion 
of the Siphonophora; and this polymorphism is explained here 
also by adaptation to different functions by the division of 
labour. Certainly before this division of labour was effected 
the colony only consisted of similar individuals, produced from 
the parent individual by gemmation ; and it was then that the 
individuality of each individual must have been most strongly 
marked ; but in course of time, in proportion as the division of 
labour was effected, this individuality was effaced, and the 
whole acquired more or less the character of a single individual 
furnished with several organs. This is what we see now-a- 
days. Therefore, in saying that the hydranth must be re- 
garded not as an individual but as a polymorphic colony, I 
do not wish by any means to say that each tentacle is a true 
individual, but only that it has been so formerly, and that it 
has retained [this character] in part even to the present day. 
I may, moreover, urge the enormous difference which exists 
between the organ tentacle and the organs of other animals 
—a foot, for example, and still more the hand of man; 
this difference is profound and primordial (principielle), be- 
cause a hand (or, in general, any organ) is not homologous 
with a tentacle, and is only analogous to it in its physiological 
function. 
Such a view as this would perfectly explain the origin of 
the organ tentacles, which would be merely the result of the 
reproduction of a Protohydra, Leuck., or rather an Archhydra, 
Hiick., by the process of gemmation. From this point of 
view, therefore, I must give the name of individual to each 
axis of cylindrical form, composed of ectoderin and endoderm ; 
and it is very remarkable that any Hydroid, however com- 
plicated it may be, appeared at first precisely in the form of a 
