TUNICATA. 



1197 



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 O'OOS'" 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 tufcs. 

 These latter commence at the surface, descend 

 vertically towards the interior, and ultimately 

 form a tuft of fine ramified rays (Jig. 775. d). 



Transverse section of the test of Salpa bicaudata, 

 magnified 30 times. (After L'owig and Kolliker.) 



1, epithelial layer; 2, homogeneous mass, with 

 nucleoli; b, fusiform and ramified concretions; c, 

 nuclei ; d, brush-like concretions. 



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. Botryllida) 

 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. 



The structure of the test of Didemnum can- 

 didum (Fam. Botryllidce) 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 O'OOG'" to 0-015'", 

 similar to those found by M. Milne-Edwards, 

 in Leptoclinum stellatum and L. maculosum 

 (fig- 774. D), 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 0-005'" O013'" 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. 

 Lowig and Kolliker 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. 



* We have been favoured by Mr. Bowerbank 

 with an opportunity of examining his valuable 

 series of microscopical preparations of ascidian tis- 

 sues, including several kinds of the spicula entering 

 into the composition of the tests. The spicula? of 

 Didemnum are spherical bodies, closely invested with 

 short, thick, blunt spines, and nearly resemble the 

 globular spiculse of Tethea, except that the invest- 

 ing spines of the latter are more numerous and 

 much finer. The tissue occupied by these closely 

 set spiculae in the Didemnum is seen in the trans- 

 parent portions of Mr. Bowerbank's beautifully 

 mounted specimens to be composed of diaphanous, 

 contiguous, irregularly hexagonal cells, the measure- 

 ments of which we have not had time to effect. 

 Numerous nucleoli are disseminated throughout the 

 tissue. 



