X 
THE VOYAGE OF H.M.S. CH ALLEN GEE . 
of the gastric cavity of the hydranth, while in Myriothela they are replaced by long, 
nipple-like processes of the endoderm, which project into the gastric cavity throughout 
its whole extent, except for a short space immediately below the mouth. These 
processes, like the more external portion of the endoderm, are formed of large, round, 
nucleated cells, but at their free extremities they are surrounded by numerous smaller 
cells which are loaded with coloured granules and are probably to be regarded as gland- 
cells. In the hydranths, indeed, of almost all Hydroids, certain endodermal as well as 
ectodermal cells would seem to possess the function of gland-cells. These endodermal 
gland-cells are chiefly developed in the walls of the hypostome. 
In almost all Hydroida the endoderm undergoes a remarkable modification in the 
interior of the tentacles. In Hydra and Myriothela the tentacles have a hollow 
tubular axis, which communicates freely with the gastric cavity, and whose endodermal 
lining is a simple continuation, in a scarcely altered condition, of that of the body cavity. 
In Garveia nutans also, the tentacle presents a continuous tubular axis, but the 
endoderm consists here of a regular series of large, clear, flat cells, piled on one another 
from the base to the apex of the tentacle, having their outer sides turned towards the 
ectoderm, from which they are separated only by the mesosarc, while their inner sides 
so encroach on the axial tube as nearly to obliterate it. In every other instance 
hitherto examined the tentacles of the hydranth are destitute of a cavity and their 
axis is occupied by a solid core of endoderm. In the great majority of cases this 
endodermal core is composed of a very remarkable tissue, which consists of large, 
cylindrical, or disc-shaped cells, which are arranged one over the other in a continuous 
series like coins in a rouleau. These cells have a distinct membrane and clear contents, 
with a central nucleus which is embedded in protoplasm, and frequently suspended by 
protoplasmic filaments to the walls. Kolliker assigns to this structure a place among the 
connective tissues, and Haeckel, under the name of “ chordal tissue.” compares it, as it 
occurs in the tentacles of certain Medusae, to the tissue of the vertebrate notochord, to 
which it bears a strong resemblance. 
In Tubularia and in Corymorpha the tentacles are disposed in two circlets, and 
the axial cells of the tentacles belonging to the proximal circlet become longitudinally 
divided, so that the axis presents the form of a tissue in which the cells have become 
multiplied laterally, and which is consequently no longer in the form of regularly 
superimposed discs. The tentacles of the distal circlet, however, in both these genera 
possess the ordinary rouleau-like axis. 
It would seem that the solid axial tissue of the tentacles is in every instance 
separated by the mesosarc, not only from the ectodermal layer of the tentacle, but by 
a duplicature of the mesosarc from the endoderm which lines the body-cavity of the 
hydranth, as was first pointed out by von Koch 1 in Tubularia , where the axial 
1 G. v. Koch, loc. cit. 
