REPORT OX THE HYDROIDA. 
7 
which extends through its entire length, and enlarges into a bladder-like receptacle at 
the base. Sir AVyville Thomson informed me that, when living, liquid was seen to 
gravitate down the stem, and collect in the basal expansion. 
The cavity which thus occupies the axis of the stem is lined by the endoderm 
(fig. 4, c), of which, however, only traces remained in the specimens. The cells of 
which this is composed seem to have contained coloured granules, and it must have been 
traversed by the longitudinal canals (cl) which, as in Corymorpha, represent the proper 
body cavity. Such at least may be inferred from the presence in Monocaulus imperator 
of the longitudinal striae which in Corymorpha are the external expression of the internal 
canals. These striae, as Sir AVyville Thomson informed me, were very well marked 
in the living animal, and they are so represented in Mr. Wild’s drawing. In the 
specimens as they reached me, however, rather faint indications of the canals were 
all that could be obtained. 
Immediately external to the endoderm lies a very remarkable tissue (figs. 4, h, 5, a, 
and 6). Indeed the presence of this tissue is probably the most striking feature in the 
histology of the Hydroid. It shows itself in the form of a fibrillated membrane whose 
most marked property is its extraordinary elasticity. The fibres of which it is composed 
take a circular course and are comparatively thick, about g^th of an inch in diameter, 
and are resolvable into finer fibrillse, but are otherwise homogeneous (fig. 6). 
Besides this fibrillated structure no other histological elements can be detected in 
the elastic tissue, which forms a transparent, colourless, and comparatively thick layer 
extending through the whole length of the stem. It is of a firm, even cartilaginous 
consistence, and its elasticity is such that separated portions of the stem-wall curl up 
forcibly on themselves. Sir AVyville Thomson describes the stem in the living animal 
as “ enormously extensile.” This extensibility is to a great extent retained in the dead 
specimen, in which the longitudinal extension of the stem is permitted by the elastic 
layer, which, when the extending force is withdrawn, brings back the stem to its previous 
length. Though the course of the fibres of the elastic layer is transverse, its elasticity 
would thus appear to exert itself in antagonising longitudinal as well as transverse 
extension of the stem. The elasticity, however, is more strongly marked transversely, 
or in the direction of the component fibres by which the stem in the dead animal is 
thrown into irregular longitudinal flutings (fig. 7). Though it is pretty certain that 
in the living animal the elastic layer must be associated with a proper contractile tissue, 
which it antagonises and controls, the state of the specimens did not allow any trace of 
this to be detected. 
Resting on the outer side of the elastic layer is the ectoderm (figs. 4, a, 5, h), consisting 
of a single layer of loosely aggregated cells, irregular in form and size, and with granular 
contents which in the preserved specimens were opaque and of a brown colour. This 
layer would seem to be in direct contact with the surrounding water, for nothing like a 
