In the Ctenoid and Cycloid fishes there is a superficial " Ganoin " layer, composed of numerous thin structureless calcified laminm, which are frequently thrown into folds, papillm or spines. The deeper substance of the scale is composed of a series of layers of a mem branous substance, each layer being composed of parallel fibres which take a different direc tion from those of the superficial and subse quent layers, so that the fibres of alternate layers cross diagonally. No endoplasts or cells are ever distinguishable among the fibres. In the deepest part of the scale these layers are entirely membranous ; but in passing to wards the surface, minute lenticular masses of calcareous matter make their appearance in the membranous substance. As Prof. Williamson justly states, these lenticular bodies are not developed between the membranous fibres and lamellm, but in them : " they corn mence as a small calcareous atom, and in.
crease in size by the external addition of new concentric laminae ; the direction of the latter not bei ngp aral lel with, or having any reference to, that o f the laminm of fibrous membrane with which they so amalgamate; thus they are not depositions from, but growths in the of connective tissue, the muscles and the vessels.
In the Vertebrata, the superficial layer of the enderon is similarly composed of indifferent tissue, and of rudimentary connective tissue; the former passing gradually into the latter, as membrane ; which growths, as they increase in size, retain their primitive tendency to assurne a lenticular form." Following the layers of the scale outwards, these isolated calcareous deposits not only enlarge, but ultimately become fused together, forming at length either a continuous calcareous mass in each layer, or presenting fissures which in some cases traverse the original lenticular calcareous deposits, in others are interstitial to them. I think one cannot but be struck with the complete analogy between the struc ture and mode of developement here described and those which I have previously shown to obtain in the calcified teg,umentary organs of the Mollusca and Crustacea. The ganoin layer corresponds very closely with the " epi dermis" of the shell or test ; the middle laminated calcified substance is formed by the fusion of concentrically laminated concretions deposited in a membranous matrix in the Fish, the Mollusk, and the Crustacean alike ; while the deep uncalcified layers of the scale are represented by the " horny " laminm which have escaped calcification in Haliotis or Unio, and still more closely by the fibrillated un calcified layers of the Crustacean test.
Structure of the enderon.— The enderon of the Invertebrata is usually entirely composed of rudimentary connective tissue or of mere indifferent tissue, consisting, in the latter case, simply of a tnatrix with imbedded endoplasts, while in the former it is produced into plates and bands, never exhibiting, however, the pe culiar bundles and elastic fibres which are met with in fully formed connective tissue.
In Paludina, according to Leydig, the pig ment masses, which lie on the surface of the ecderon, are connected by " clear large cells, with a small parietal nucleus." From their occurrence, wherever in the higher animals connective tissue is found, Leydig calls them "Binde-substanz-zellen"—"Connective tissue cells ; " but, as he himself points out, they fre quently contain carbonate of lime, and their relation is rather, like that of the similar cells in Piscicola, to fat.
A wonderful complication of structure is attained by the skin of the Cephalopoda. Ac cording to H. Milner *, who has recently made some careful investigations on this subject, there lie beneath the cellular ecderon in these ani mals : 1st, a fibrous layer, usually colourless, but occasionally white and glittering. 2nd, the layer with the chromatophora (ride inf.). 3rd, beneath these a peculiar layer, which gives rise to the colours produced by interference, the metallic lustre, and intense whiteness of many localities. It consists frequently of regular plates, which evidently proceed from nucleated cells. 4th, deeper still lie the larger bundles * Bericht, &e. Zeitschrift fur Wiss. Zoologie. 1853.
we trace it inwards, developing its elastic ele ment to a greater or less extent, and acquiring a more or less distinctly fascicular arrangernent of its collagenous element. In the higher Vertebrata, these bundles are usually disposed as an irregularly felted mass ; but in Fishes and Batrachia, they form regularly super imposed horizontal strata, tied together by perpendicular columns, which penetrate the interspaces of the bundles, and spread out into the irregular connective tissue on the deep and superficial surfaces of the stratified mass (fig. 319. A). On the addition of acetic acid, it is seen that the boundaries of the strata are formed by irregular bands of elastic tissue, in which the remains of the primitive endo plasts may be seen (as in fibro-cartilage), whose strongest fibres are horizontal, though they send out others irregularly in all direc tions. The perpendicular columns are likewise composed of bundles of pale elastic fibres (fig. 319.n), and if the intersection of the horizontal with the vertical divisions be carefully examined, it is seen that the former are, as it were, given off by the latter, which thus gradually break up and thin out, terminating above and below in the elastic fibres of the unstratified super ficial and deep layers. A horizontal section of this portion of the enderon presents a very peculiar appearance, the transparent vertical columns looking like radiating spaces, as which they were, in fact, at first described.