The third layer forms the principal mass of the test of this species. It is the seat of the numerous and large arteries, which, arising from the heart, traverse it in every direction, having brush-like ramifications, that penetrate almost to the exterior surface, and then ap pear to pass into other vessels that accom pany them in their course. This layer is tbrmed of large cells, besides a clear homo geneous substance, which is a continuation of the principal substance of the second layer ; besides these there are locally distributed crys tals, nuclei, and pigment-cells.
The large cells, which R. Wagner previously thought to be cartilaginous, are of a peculiar nature, and resemble no other animal cell hitherto known, except perhaps those of the chorda dorsalis of some animals. The most re markable character of these cells is their size, which varies from to 0.05m, the aver age being 0.02'" to 0.03m. Their form is spherical, pyriform, or elliptical ; their con tents diaphanous and quite destitute of nucleus or granules ; and their membrane delicate, smooth, and of an equal thickness throughout.
The smallest cells are irregularly dispersed in the homogeneous fundamental mass that is common to the second and third layers. The larger cells are arranged closely together to wards the exterior surface of the layer, pre senting a very regular cellular tissue with very little intermediate substance; but, immediately beneath the external surface of the envelope, the cells are rather more distant one from another, and the intermediate tissue more visible. The crystals and pigment-cells before mentioned, are present only in the outer part of this third layer ; the former are acicular, about 0.0015m in length, occupying in com pact masses the intercellular intervals ; the latter are yellow, and filled with somewhat large granules, and surround in particular the extreme ramifications of the vessels. The nuclei, lastly, are similar to the large nuclei of the second or intermediate layer, and are everywhere present between the large cells in considerable numbers.
When slices of the test are treated with hy drochloric acid, the crystals of the second and third layers quickly disappear ; treated with a solution of soda, the epithelial cells, the nuclei, the pigment-cells, and the vessels are dissolved. The fundamental homogeneous sub stance of the second and third layers and the large cells are not dissolved, nor do they suffer any modification.
In Phallusia monachus the large cells mea sure from to and are more dis. tinctly separated from one another than in the preceding example. The nuclei of the homo geneous substance are few, and generally fiisi form or even ramified ; at the external surface of the envelope they are mixed up with a great number of minute yellow pigment-cells and pigmentary granules, as well as with acicu lar crystals and very minute crystalline concre tions ; all of these being in the greatest num bers in the neighbourhood of the extremities of the vessels. In one specimen of this spe
cies MM. Lowig and Ki.illiker observed, that in the interior of the third layer no cells could be distinguished ; ultimately, however, they distinctly saw well defined cavities or lacunae, which were evidently vestiges of cells that had been more or less completely fused with the intermediate homogeneous substance ; and the traces of these lost cells were found to be more and more distinct towards the band of perfect cells in the surface of the test.
In Phallusia sulcata the large round or ellip tical cells, without nuclei, have a diameter of 0.0 In Phallusia gclatinosa a very peculiar forma tion was observed by the experimenters. In one specimen the soft, gelatinous substance of the envelope exhibited no trace of cells throughout its thickness, but its mass was principally composed of a homogeneous sub stance similar to that of the other Aseidice. In another individual they observed some few and indefinite remains of cells. In both spe cimens they found, as in the other species, vessels and nuclei, the latter for the most part round and measuring 0•002'", in the homo geneous substance. In the individual desti tute of cells, there was also in the exterior part a very large quantity of acicular crystals and yellow granules, the latter frequently re sembling the nuclei with large coloured nu cleoli. The tissue in all the Aseidice examined, when chemically treated, behaved in a similar manner to that of Phallusia We have observed that the test of Bottenia renifornis (preserved in spirit) presents a ho-, mogeneous structure crowded with " nuclei " and bloodvessels, and only occasionally does any trace of cellular tissue present itself, in which case the cells are very minute, poly gonal, and compressed. Mr. J. Quekett has detected calcareous spiculm in the test of this species. They are situated towards the ex terior, and are very numerous and excessively minute. Their form is usually cylindrical, with triradiate or 4-5-fid extremities.* The composition of the test of Clavellina lepadiformis corresponds in all respects to that of the above mentioned Ascidice; certain parts in the same individual having a more particular resemblance to one species, and others to another. The test, however, is quite destitute of bloodvessels. Transverse sections of the stalk of the Clavellina and of the excrescences that spring from it, exhibit a tissue composed of round or elongated non-nucleated cells, almost destitute of inter mediate tissue, and arranged very close to one another.