I have hitherto described only the structure of the middle, most characteristic portion of the Ascidian test ; it is next necessary to notice the inner and outer surfaces ; the former of which is ordinarily said to be covered by a cellular epithelium, the latter by a more or less structureless horny epi dermic layer.
The so-called epithelium is, I believe, in all cases merely the innermost unmetamorphosed layer of the ecderon, corresponding with the rete Malpighii of the " epiderniis " of higher animals. As the Ascidian integument is ordi narily examined (i. e. in spirit specimens), it is in the condition of the macerated integu ment of one of the higher animals, and just as the " epidermis " of the latter may or may not, if stripped off, bring away with it the deepest layers of the rete, so the Ascidian test, when detached from the outer tissue, may or may not retain the corresponding structure.
The horny so-called " epidermis," on the other hand, is a structure well worthy of at tention, as a similar element is, as we have already seen, to be met with in the widely different integuments of other Mollusca. In all Ascidians I have found the outermost sur face to be formed by a structureless homoge neous layer, which contains less cellulose than the subjacent tissue, and often has a brownish horny aspect. In many Salpw, Phallusiw, and Cynthim, this outer layer constitutes merely a tough wrinkled investment. In others (Syniiicum, Boltenia) it is prolonged with the subjacent layer into spines and processes, but without being much thickened. In other Boltenim again, and in various Cynthi, it is greatly thickened, and almost by itself consti tutes large spines or even tesselated plates. In Cynthia papillata (fig. 314. a), the whole outer surface of the test is covered with spines (a), whose bases expand into polygonal plates, which strongly resemble the spines of the Rajidx, to which reference will be made below. The brown substance here appears to have invaded the subjacent tissue, leaving spaces for the pre-existing endoplasts, so as to give rise to a structure precisely resembling the bone of the Plagiostornes, while to com plete the resemblance, the pointed extremity of the spine is marked by lines which pass from its central cavity, parallel with one another, to the surface. I am not sure that there are tubes, but otherwise the appearance is exactly that presented by the pseudo-den tine of the integumentary spines of the skate.
It would appear, according to Milne Edwards, and the late observations of Krohn, that the rudiment of the mantle exists in the ovum of Phallusia before the cleav age of the yolk commences, as a structureless pellucid coat, containing solitary or aggregated greenish cells ; and it would seem as if the outer structureless layer with which we are at present concerned, arose from this coat, while the main thickness of the mantle is the product of the metamorphosis of the subse quently developed ecderon.
From all that has been said, I think it re sults that the Ascidian test is formed from the eederon of the animal by a process of conversion which consists in the deposit, through its periplast, of cellulose, and a coin cident morphological change which may re sult in the production of a tissue essentially resembling either cartilage, bone, connective tissue, or even dentine ; and that, therefore, an attentive study of the integurnent in this class alone is sufficient evidence that mere structure is no proof of the ecderonic or enderonic nature of any given organ.
Integument of the Vertebrata. — In these animals there are two classes of integumen tary organs, differing in structure, chemical composition, and mode of development. These are, 1st, the horny and glandular tegu mentary organs produced by the conversion of the cellular ecderon ; and 2nd, the ca/crfied tegtunentary organs which appear very fre quently to be developed by a process of excretion.
1. Conversionaryhorny organs.—If a section be made of the integument of any mammal, it will be seen to be composed, leaving out of view its various appendages, of two principal portions, the enderon or derm, and the ecderon or epidermis. The latter, separated by a more or less distinct transparent line from the for mer, is internally composed of a homogeneous soft substance, in which are dispersed nume rous oval or rounded endoplasts, set more or less perpendicularly to the surface of the enderon. Further outwards, they gradually become more distant and a cavity is de veloped round each, so that the ecderon becomes distinctly' cellular. Still more ex ternally the cellular periplast becomes changed in composition, being converted into a denser horny substance, and the change usually takes place so suddenly that the horny external portion (epidermis) is sharply marked off optically, and can be readily separated me chanically and chemically, from the internal unaltered soft porton, the rete Malpighii. The cell cavities at the same time become flattened, and by degrees almost obliterated, apparently by the pressure of the subjacent growing tissue ; but the endoplasts remain, and may always be detected if the horny layers are distended and rendered transparent, by the action of acids or alkalies. The horny stratum of the epidermis is therefore the result of the conversion of the walls or periplast of a whole layer of the cells of the ecderon into horn.