T HE VOYAGE OF H.M.S. CHALLENGER. 
viii 
Now, the whole of the nutritive zooids or hydranths with the connecting coenosarc 
may be regarded as forming a distinct system, while the generative zooids or gonophores, 
along with such parts as they may be provided with for protection or otherwise, will form 
another. The former will constitute the trophosome of the colony, the latter the gonosome. 
The zooids, which have just been referred to the trophosome and gonosome, have been 
found in all Hy droids which have been adequately studied, with the exception of some free 
Medusae in which no independent trophosome is present (see below, p. xxviii), of Hydra 
in which the protection of the generative elements is not assigned to distinctly differen- 
tiated zooids, and of the somewhat enigmatical Protohydra of Greeff, which according to 
that author is a Hy droid, presenting a condition of extreme simplification, being destitute of 
tentacles and multiplying only by division of its body. 1 In some, however, there occurs 
an additional set of zooids, whose functions, though scarcely yet ascertained with certainty, 
would seem to be in some cases chiefly that of the defence of the colony, as may be 
assumed of the “ spiral zooids ” of Hydractinia, and the “ dactylozooids ” of the Hydro- 
corallia ; and in other cases that of the prehension of nutriment, as is probably the case 
with the nematophores or sarcostyles of the Plumularinse and of a few other groups. 
It will thus be apparent that the zooids of the Hydroid colony, with their various 
forms and definite functions, are the expression of a physiological division of labour, 
while as we advance towards higher groups of the animal kingdom we find the various 
kinds of physiological work assigned, not to different zooids, but to different organs 
which have lost the independence which more or less characterises the zooid. 
IV. Histology of the Fundamental Layers. 
What has now been said will serve to indicate the salient points of the morphology 
of the Hydroida. For an adequate conception, however, of Hydroid organisation, a more 
intimate knowledge of structure will be necessary, and this will be best acquired by a 
detailed examination of each of the fundamental tissue layers. 
1. The Endoderm. 
The endoderm consists generally of a single layer of prismatic cells, whose bases rest 
externally on the mesosarc, and whose opposite ends bound the common cavity of the 
colony. Those ends of the endodermal cells, which thus lie free in the nutritive cavity, 
are each provided with a simple flagelliform cilium, by whose vibrations the contents of 
the cavity are kept in constant motion. 
In Myriothela the endoderm of the whole of the main cavity of the body forms a 
thick layer composed of many cells in depth. The cells, which in Myriothela form the 
1 R. Greeff, Zeitschr. f. wiss. Zool., Bd. xx., 1869. 
