The separation of the ecderon (or test) from the enderon (outer tunic) takes place with great readiness in some Ascidians, as Phallusia,&c.; while in others, such as Boltenia, many Cynthim, Salpre, tire., it can be effected with considerable difficulty, or not at all, and this difference has even been raised to the rank of a zoological distinction, the Ascidians having been in consequence divided into Monochito nida and Dichitonida. I believe, however, that in all those Ascidians whose test is un provided with vessels, it is, normally, closely adherent to the outer tunic, and I am inclined to think that this is equally true even of those forms, such as the Phallusim, in which in preserved specimens the test and outer tunic are so commonly found detached from one another. Here, however, as the test is pro vided with an abundant vascular supply pro ceed ing from one point of the body, it may normally become separated elsewhere. Care ful examination of fresh specimens can alone decide this point.
The shnplest form of Ascidian test is that presented by the Salpm. Here, we have merely a gradual growth of the periplast and a deposit of cellulose within it, the endoplasts either remaining as such or becoming sur rounded by cell walls. The resulting tissue, in fact, is identical with cartilage, if we suppose cellulose to have taken the place of chondrin.
In the Pyrosomata, the test has a struc ture which, on the one hand, resembles bone, on the other some forms of fibro-carti lage. The endoplasts, in fact, have become surrounded by cell walls, which are produced into long, frequently anastomosing processes (fig. 314. o); these retain their animal com position, while all the immediate tissue is strongly impregnated with cellulose. This is the fundamental structure of the test in the Phallusim and Clavelinm also ; but here an additional complication results from the development in the substance of the test of a series of rounded cavities, which gradu ally enlarge until they ahnost come into con tact, and give rise to a spongy texture. The intervening septa at the same time frequently become obscurely fibrous (fig. 314. c). Now these " vacuolm,' whose origin and nature ap pear to me to show their identity with the " cancelli " of bone developed from cartilage, have been described by 1,6 wig and Kolliker, anti by Schacht as cells ; and the latter has even stated that they possess a nitrogenous lining membrane. This is, however, a mistake, arising from the imperfect operation of the reagents by which the cellulose is detected ; it is simply less abundant close to the cavities of the vacuolm, but may with care be demon strated to exist up to their very edges.
Botryllus, Syndicum, Syntethys, Boltenia, and the Cynthim, present a new series of ap pearances : here the periplast of the ecderon is metamorphosed into fibres which, however, are not composed of pure cellulose, but of a nitrogenous substance impregnated therewith.
In Syndicum the test is soft,and presents very much the structure of some forms of rudimen tary connective tissue. We find, in fact, a more or less distinctly fibrillated blsis with scattered endoplasts ; some of these are in vested by round granulous nitrogenous cell walls, while in others the cells are spindle shaped and prolonged at each end into fibres (representing thus the elastic element of or dinary connective tissue), or they may be stellate. Botryllus, Syntethys, and Boltenia, present a similar structure, varying, however, in the extent to which the nitrogenons eell walls on the one hand, and the periplast im pregnated with cellulose on the other, have undergone development. Thus the periplast is broken up into very obvious fibres in Bo tryllus, while in Boltenia the fibrillation is pale and indistinct. On the other hand, I have nowhere met with so great a develop ment of the nitrogenous cell-wall as in Syndicum.
In Boltenia a more or less distinct lamina tion makes its appearance in the test, and this peculiarity, as well as the fibrous structure altogether, attains its maximum in the Cyn thim. In Cynthia papillata, for instance, the middle substance of the test is composed of numerous, very obvious lamina', which con sist of fibres directed alternately parallel with, and perpendicular to, the surface of the test (314. B.) At first sight, they appear as Ldwirt and Ktilliker have described them, to be decussating sets of longitudinal and radiating ; but on a careful examination of their sections I invariably found that the apparently radiating fibres bend round as they approach the apparently longitudinal set, and in fact pass into the latter. The longitudinal bands are, however, no thicker at one end of a section than at the other, so that the trans verse fibrils cannot be merely given off from them. A transverse section, again, exhibits the same appearances as a longitudinal one; so that 1 think the fibres must in reality have a more or lesii regularly circular arrange 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 cen doplasts ?) are scattered through the midst of the " transversely fibrous " spaces. In Cynthia pomaria, Lowig and Kiilliker 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.