A. Hydrida.
T
THE Hydras, as a rule, are not coloured in our sense of the term; that is to say, they are of a general uniform brown colour. But in one species, H. viridis, the endoderm contains granules of a green colour, which is said to be identical with the green colouring matter of leaves (chlorophyll). This does not occur in all the cells, though it is present in most. The green matter occurs in the form of definite spherical corpuscles, and these colour-cells define the inner layer of the integument (the endoderm), and render it distinct.[22] That portion of the endoderm which forms the boundary of the body-cavity has fewer green corpuscles, but contains irregular brown granules, thus roughly mapping out a structural region.
We thus see that even in so simple a body as the Hydra the colouring matter is distributed strictly according to morphological tracts.
B. Tubularida. The Tubularian Hydroids are the subject of an exhaustive and admirably illustrated monograph by Prof. J. Allman, from which the following details are culled. These animals are with few exceptions marine, and consist either of a single polypite or of a number connected together by a common flesh, or cœnosarc. Some are quite naked, others have horny tubes, into which, however, the polypites cannot retreat. The polypites consist essentially of a sac surrounded with tentacles; and one of their most striking characters is their mode of reproduction. Little buds (gonophores) grow from the cœnosarc, and gradually assume a form exactly like that of a jelly-fish. These drop off, and swim freely about; and are so like jelly-fishes that they have been classed among them as separate organisms.
The Tubulariæ are all transparent; and in them we find structural colouration finely shown, the colour, as is usual in transparent animals, being applied directly to the different organs.
Writing of the colour, Prof. Allman says: "That distinct secretions are found among the Hydroida, and that even special structures are set aside for their elaboration, there cannot now be any doubt.
"One of the most marked of these secretions consists of a coloured granular matter; which is contained at first in the interior of certain spherical cells, and may afterwards become discharged into the somatic fluid. These cells, as already mentioned, are developed in the endoderm;[23] in which they are frequently so abundant as to form a continuous layer upon the free surface of this membrane. It is in the proper gastric cavity of the hydranth and medusa, in the spadix of the sporosac, and in the bulbous dilatations which generally occur at the bases of the marginal tentacles of the medusæ, that they are developed in greatest abundance and perfection; but they are also found, more or less abundantly, in the walls of probably the whole somatic cavity, if we except that portion of the gastrovascular canals of the medusa which is not included within the bulbous dilatations.
"In the parts just mentioned as affording the most abundant supply of these cells, they are chiefly borne on the prominent ridges into which the endoderm is thrown in these situations; when they occur in the intervals between the ridges they are smaller, and less numerous.
"The granular matter contained in the interior of these cells varies in its colour in different hydroids. In many it presents various shades of brown; in others it is a reddish-brown, or light pink, or deeper carmine, or vermilion, or orange, or, occasionally, a fine lemon-yellow, as in the hydranth of Coppinia arcta, or even a bright emerald green, as in the bulbous bases of the marginal tentacles of certain medusæ. No definite structure can be detected in it; it is entirely composed of irregular granules, irregular in form, and usually aggregated into irregularly shaped masses in the interior of the cells. It is to this matter that the colours of the Hydroida, varying, as they do, in different species, are almost entirely due.