It is only towards the upper extremity of the stalk that the cells become more and more separate, and even disappear little by little, as happens in some Ascidice, and give place to an intermediate homogeneous substance, bear ing a quantity of nuclei. In the largest and superior moiety of the test a peculiar structure is found. Externally is a dense, hut not thick, layer of delicate cells, which are very difficult to recognise, and measure about 0-02"'. Be tween the cells, and immediately at the surface, are crystals of carbonate of lime, scattered nuclei (measuring and large round fatty granules. Interiorly to this occurs a still thinner lamina, composed of a trans parent, colourless, homogeneous substance, with infinitely minute pale granules. Next, there is a layer of round granules or vesicles. These are spherical, measuring 0.000-bm, and even their surface is smooth or granulated; the largest are placed in the middle, the smallest at the exterior; they appear opaque, and like starch or fat granules. With solution of iodine, they be come yellowish, without presenting any trace of blue, and are probably fat grains.
Succeeding to these, a thick layer presents itself, homogeneous, diaphanous, with some few minute spherical nuclei, which, the nearer they approach towards the interior, contain more and more colourless granules. Lastly, quite at the interior surface, is a thinnish, com pletely diaphanous substance, of equal thick ness throughout, with spherical granular nuclei, measuring about Treated with soda and with hydrochloric acid, the crystals, nuclei, and fat granules of the test of the Clavellina disappear ; the large cells, on the contrary, and the homogeneous substance, with its scattered granules, remain perfectly unaffected, proving the identity of the chemical composition in Ascidia and Cla The Salpa maxima does not contain, in its gelatinous envelope, any trace of cells similar to those of the Phallusia and Clavellina. It is for the most part composed of a homogeneous, clear, diaphanous substance. Towards the interior surface, the several elements are not so clearly arranged as in the middle and external layers. In the innermost layers, a multitude of very minute granules are pre sent ; in the others there are little round nuclei, nucleated cells, and spherical or star shaped crystalline concretions. These latter are very regular, and formed of 3-7 straight rays, springing from a centre (fig. 774. c). They are composed of a single or manifold series of granules, which, as they approach the exterior, increase or lessen in number, but always di minish in size. These do not appear to be car bonate of lime, not being soluble in hydro chloric acid ; and their behaviour with muriate of barytes shows that they are not formed of sulphate of lime ; probably they are siliceous.
The granules and nuclei are dissolved by boil ing in solution of soda ; but the homogeneous substance, composing the mass of the tunic, remains unchanged.
The test of Salpa bicaudata is essentially the same, in the nature of its composition, as that of the Salpa maxima, the fundamental mass of the gelatinous envelope being composed of the homogeneous substance. It differs, however, in the elements contained in this tissue, and by the presence of a simple layer of epithelial cells, covering it on the interior surface of the test. In the interior layer of the homoge neous substance there are granulated vesi cles, having a diameter of 0.003"' to 0.004"/, sometimes having the appearance of nuclei, and sometimes that of cells. In the middle part, here and there, are scattered round or fusiform nuclei ; and in the exterior layer are little crystals, round nuclei, and peculiar concretions, similar to those of Salpa maxima. Some of these concretions are small, elegantly ramified, and disposed horizontally ; others are larger, ramified in a brush-like form, and appearing to the naked eye as white tufts. These latter commence at the surface, descend vertically towards the interior, and ultimately form a tuft of fine ramified rays (fig. 775. d).
Magnified 350 times, these two sorts of con cretions are seen to be composed of opaque granules of different sizes. The chemical composition of the test of this Salpa is iden tical with that of the preceding species.
In the Pyrosoma giganteum, the common en velope of the individuals is also formed of a homogeneous and structureless substance. In its interior are scattered here and there round nuclei, and some ramified cells, similar to those of the loose cellular tissue of the em bryos of Mammifers, for example, to that of the gelatine of Wharton. Both of these latter elements disappear when treated with soda ; but the homogeneous substance al together resists the action of the soda, and remains intact.
The Diazona violacea (Fam. Botryllithe) possesses, in the gelatinous mass of the com mon test, a diaphanous, structureless sub stance, quite destitute of cells. In the ex terior layers are crystals and concretions of carbonate of lime, vesicles with violet-coloured granules, fat granules, and, particularly to wards the interior, a great quantity of minute round vesicles (nuclei). Treated with hy drochloric acid and with soda, the crystals, nuclei, and pigment-cells are dissolved, but the homogeneous substance remains un changed. Moreover, after having been a long time exposed to the influence of the alkali, and although the pigment-cells have disap peared, some portions of the exterior parts retain a pale violet tint, giving evidence of the presence of undissolved colouring particles, and of an amorphous colouring matter per vading the mass.