A careful examination of a number of species of the Ascidian genera Boltenia, Cynthia, Molgula, Phallusia, Syntethys, Aplidium, Pyrosoma, and Salpa—including, therefore, every modification of the type, has led me to the following conclusions with regard to the structure of the mantle. The investigation was made with a full knowledge of what had been done by Ldwig and Kolliker and by Schacht (p. 34), and I have only ventured to differ from them upon strong evidence.
1. In the most gelatinous forms of the test, as in Syntethys and Salpa, it consists of a soft homogeneous or delicately striated basis, through which round nucleated cells (nuclei of Kolliker and Lowig) are scattered. These cells present no ramifications, and the presence of cellulose is demonstrated with very considerable difficulty. When the iodine solution is added the whole mass becomes coloured yellowish-brown, the nucleated cells taking rather a deeper tint than the rest. The addition of sulphuric acid slightly contracts the whole substance, and if used with care gives the edges the characteristic blue tinge. The cellulose is evidently diffused through the intercellular nitrogenous basis ; for the first evidence of the operation of the sulphuric acid is seen in a slight diffused, even green shade, which is produced by the incipient blue re-action of the cellulose mingling with the existing yellow-brown colour. As the action of the test goes on, the edges of the membrane become deep blue, while the green tinge passes insensibly into the blue on the one side and into the yellow on the other.
As Schacht justly points out then, the substance of the test is not pure cellulose but cellulose deposited in a nitrogenous membrane. It exists in the same condition as the calcareous salts in bone, or as the chondrin in cartilage.
Substituting cellulose for calcareous salts, the structure of the test of Salpa is exactly that of the bone of plagiostomous fishes (Leydig, Beitrage zur Anat. d. Rochen. Haie, 1852).
2.Pyrosoma has a firmer test, which contains far more cellulose. This is more readily detected by the iodine and sulphuric acid, but in its relation to a general nitrogenous basis precisely resembles that of Salpa.
The nucleated cells differ from those of Salpa in being thrown into very long processes which meet and unite—-just like those of Volvox as described by Mr. Busk. On the other hand they assume exactly the appearance of bone corpuscles— though the processes are generally straighter and are rarely branched.
Making the same substitution as before, we have in the test of Pyrosoma a structure comparable to that of the lamina papyracea of the ethmoid bone.
3. In the Phallusiæ there is an indistinctly fibrillated basis, containing a large amount of cellulose in all essential respects resembling the foregoing. Nucleated cells, provided with irregular processes, are scattered through this substance.
The large cells described by Löwig and Kölliker and by Schacht are not cells at all but are vacuolæ—very probably produced, like the cancelli of ordinary bone, by interstitial absorption. There is no lining membrane like that described by Schacht. With care the walls may be coloured deep blue to their very edges. The appearance of fibres is produced by the striation which runs through the whole mass, and is especially distinct upon the walls of the cavities. It exists after the action both of sulphuric acid and of caustic soda.
Lastly, the resemblance to perfect bone is completed by the canals which are hollowed out in the substance of the test for the vessels (or rather prolongations of the outer tunic, which is what they really are). In the walls of these canals I have frequently seen the nucleated cells projecting just as Kölliker describes them in the “medullary canals” of developing bone (Mikroskopische Anatomie, p. 369).
The spiral fibres described by Schacht are the muscular fibres surrounding the wider portions of the vessel-like prolongations.
Finally, with regard to the relations of the cells to the cellulose—anatomically and physiologically—I do not see any force in the distinction attempted to be established by Schacht between animals and plants. The nucleated cell of the Ascidian tunic answers exactly to the primordial substance of the plant. The cellulose is deposited outside both. The amount of nitrogenous matter mixed up with the cellulose deposit appears to be a mere question of degree—and the nature and existence of an intercellular substance in the vegetable kingdom are still matters too much disputed to be good grounds of distinction.
The physiological theory of Löwig and Kölliker, that the cellulose of the Ascidians is derived from the Diatoinaceæ upon which they feed—is incompatible with the fact (Annals of Nat. Hist, Aug., 1852) that the larval Ascidian contains cellulose before any of its organs are developed.
To examine the test of an Ascidian for cellulose, I find the best way to be, to take a very thin section and moisten it with a strong solution of iodine in iodide of potassium. After being thoroughly impregnated with the iodine, the superfluous fluid should be drained off, and the segment carefully blotted with the finger [or hair pencil], A handkerchief or blotting-paper may readily give rise to error by leaving behind small fragments of vegetable fibre. A drop of sulphuric acid, as strong as can be procured, should now be added. If much cellulose is present a deep blue colour will appear immediately, beginning at the edges of the slice ; if there be but little, the colour will not appear for some time. The application of the test requires some care ; and while its success is most valuable evidence of the presence of cellulose, its failure is not by any means negatively conclusive, unless the experiment has been frequently and carefully repeated.
