The structure of the test of Didemnum can. didunz (Fam. Bat is quite different from that met with in any of the Tunicates before mentioned. The white substance in which the individual animals are lodged, which has been figured also by Savigny, apparently pre sents only some white star-shaped bodies (fig. 774. A), measuring 0.006"/ to 0.015'", similar to those found by M. Milne-Edwards, in Leptoclinum stellatum and L. maculosum (fig. 774. n), except only that the former are of a more rounded form, and are provided with shorter and more numerous points.* But, on being treated with hydrochloric acid, this substance has quite another appearance. The white colour quickly disappears, bubbles of gas being freely given off; and, on ex amination with the microscope, there is seen in the yellowish, transparent membrane that remains, a fundamental homogeneous sub stance, in which are scattered round and elongated cells, of in di ameter, and some minute granular masses. At first sight these cells appear to be analo gous to the large cells found in the tests of Ascidia and Clavellina, being, like those of the latter, non-nucleated, indistinctly marked by a pale, delicate contour, and having perfectly liquid contents. But by boiling with soda they are quite dissolved, whereas the homo geneous substance remains unchanged. MM. Ldwig and Kiilliker found also, that under the influence of hydrochloric acid each of the star like corpuscles showed itself not to be a simple concretion, but, losing its rays little by little, became a cell filled with lime, and ultimately appeared as a colourless, empty cell, quite similar to those above described. They add that they could not discover how these curious cells, filled with lime, and fur nished with calcareous rays externally, were formed ; but they thought it probable that they were originally large cells, full of liquid, which became gradually occupied with lime, until ultimately the membrane of the cell became incrusted, and the lime deposited on its external surface.
The common test of Aplidium gibbulosunz presents a homogeneous substance with some scattered nuclei, and a great quantity of round cells, with very delicate membranes, measur ing to WO l3m, and even to 0-02m. Those in the interior contain only a liquid ; but the nearer they approach the exterior, the more are they found to contain calcareous concretions ; and, lastly, there are cells per fectly incrusted, but without appendages. Acted upon by soda and by hydrochloric acid, the homogeneous substance alone remains, the rest is dissolved.
In the "common body " of Botryllus vio laceus are some remarkably incrusted cells, similar in some respects to those of Didemnum. Some of these are perfectly round, with a diameter of 0.009m, and, as those of Aplidiuni, are filled with calcareous concretions ; others, for the most part pyriform or fusiform, have one or two pointed, colourless prolongations, 0.006'"-0.009" in length, and are organic in substance ; others, lastly, round or tetra hedral, have even three or four of these prolongations, which are often of a similar length, and are regularly disposed, but just as often are of different sizes, and without sym metry (fig.774. B).* These prolonged cells
are probably analogous to the round cells in Dideninum, that have lime in their interior and calcareous deposits externally, and may even be compared with vegetable cells (pollen granules, spores, &c.), bearing external de posits. Were these cells incrusted, they would form star-like bodies, similar to those of the Didenznum.
Professor M. Edwards observes, that in Lep toclinum the substance of the tissue is crowded with calcareous granules, which, seen with an ordinary lens, appear to be little spherical con cretions, but which are aggregations of little pyramidal crystals, united by their base, so as to represent a many-rayed star, surmounted on each of its faces by a group of other similar, but smaller, rays (see fig. 774. D).
The structure of the common test of the Botryllus polycyclus is peculiar and quite dif ferent from those described above. In the exterior parts of the common mass the struc ture resembles that ordinarily found in com pound Ascidians, being of a clear and homo geneous substance, with some nuclei and crys tals; but in the interior distinct fibres are found, by the side of the nuclei. These fibres are of two kinds ; some, the least numerous, are long, extremely pale and delicate, too fine to be measured, and, crossing one another in all directions, form elegant sinuosities ; others, less numerous, are short, to 0.03"/ in length, larger, opaque, and variously curved ; in a word, they resemble certain nuclei transformed into fibres (Kernfasern). Like the homoge neous substance of the exterior and interior parts, these fibres resist the action of hydro chloric acid and of soda ; and consequently, since they are incontestably organic, they are composed of non-azotised substance.
There are also some round points, visible to the naked eye, dispersed in the common in tegument of this Botryllus. Some are white, generally situated towards the interior of the test, and appear under the microscope as groups of granules or spiculm ; they are in soluble in a solution of soda, or in hydro chloric acid, and are probably siliceous, like the concretions in Salpa. The other spots are violet-coloured, or reddish, and are most abundant in the external layers near the groups of individuals, yet sometimes also they are present in the interior parts of the common mass. Seen under the microscope, they appear as pyriform, round, or elongated vesicles, bearing a reddish colouring matter, contained probably in the cells, and are at tached to the extremities of the ramifying canals that traverse the mass in every direc tion. These vesicles are the germs of new individuals.* The coriaceous test of the Cynthice (Fam. Ascidiadce) presents a composition still more remarkable than that of the Botrylli. In Cynthia papillata, the fibres that constitute a large proportion of the test, are in some parts so much developed, that they may bear compari son with the fibrils of any fibrous tissue found in vertebrate animals.