The animal matter, or sarcode, or protoplasm of sponges falls into three layers, just as we find the primitive embryo of the highest animals; and just as the middle membrane of a mammalian ovum develops into bone, muscle and nerve, so the middle membrane (mesosarc) of the sponges develops the hard skeleton, and in this most important part we find the colour cells prevail. Sollas, one of our best English authorities upon sponges, writes, "The colours of sponges, which are very various, are usually due to the presence of pigment granules, interbedded either in the _endosarc of the flagellated cells_, or in the mesodermic cells, usually of the skin only, but sometimes of the whole body."[21] We can, then, appeal most confidently to the protozoa as illustrating the morphological character of colouration. [Illustration] [19] Leidy. Rhizopoda of N. America, p. 16. [20] Haeckel. Die Radiolarien, Berlin, 1862. [21] Sollas. Spongidæ. Cassell's Nat. Hist. Vol. vi., p. 318. CHAPTER IX. DETAILS OF COELENTERATA. I. HYDROZOA. _A. Hydrida._ 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 coenosarc. 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 coenosarc, 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.