TEGUMENTARY ORGANS. 



495 



ment around the centre of the spaces occupied 

 by the radiating bands, the apparently longi- 

 tudinally fibrous bands arising merely from 

 the decussation of these circular fibres. 

 Great numbers of granular corpuscles (en- 

 doplasts ?) are scattered through the midst 

 of the " transversely fibrous " spaces. In 

 Cynthia pomaria, Lo'wig and Kolliker de- 

 scribe peculiar " cells " in the inner layer of 

 the test, consisting of such corpuscles sur- 

 rounded by a thick circularly fibrous wall, 

 and the existence of these bodies appears to 

 be additional confirmation of the view I have 

 taken as to the mode in which the fibres in 

 Cynthia papillata are disposed. If in the 

 latter, the fibres were disposed more closely 

 around particular corpuscles, the test would, 

 in fact, break up into just such circularly 

 fibrous cells. 



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 " epidermis " of higher 

 animals. As the Ascidian integument is ordi- 

 narily examined (z. 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 Salpae, Phallusiae, and 

 Cynthiae, this outer layer constitutes merely 

 a tough wrinkled investment. In others 

 (Synoicum, Boltenia) it is prolonged with 

 the subjacent layer into spines and processes, 

 but without being much thickened. In other 

 Bolteniae again, and in various Cynthiae, it is 

 greatly thickened, and almost by itself consti- 

 tutes large spines or even tesselated plates. 

 In Cynthia papillata (fig. 314. B), 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 

 Rajidae, 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 Plagiostomes, 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 ecderon 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 integument 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 Vcrtebrata. 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 calcified 

 tegumentary organs which appear very fre- 

 quently to be developed by a process of 

 excretion. 



I. 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 portion, 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 



