1196 



TUNICATA. 



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 spiculae 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, 

 O'Ol'" 0-014/", almost destitute of inter- 

 mediate tissue, and arranged very close to one 

 another. 



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 Ascidits, 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, but not thick, 

 layer of delicate cells, which are very difficult 

 to recognise, and measure about Q'Q2" f * Be- 

 tween the cells, and immediately at the surface, 

 are crystals of carbonate of lime, scattered 

 nuclei (measuring G'002^ 7 ), 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 G'0005"', 

 O'000-l/", and even 0-005'"; 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 O'OOS'". 



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- 

 vellina. 



* Several specimens are figured by Mr. Quekett in 

 the Descript. and Illustr. Catal. of the Histological 

 Series in the Mus. Royal Coll. Surgeons, 1849, plate 

 xvii. fig. 13. 



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 



Fig. 774. 



A. Incrusted cells of the fundamental mass of 

 Didemnum candidum. a, unaltered cells ; b, a cell 

 the lime of which has been nearly extracted by 

 means of hydrochloric acid. (After Lowig and 

 Kolliker}. 



B. Incrusted cells of Botryllus violaceus. a, sphe- 

 rical cell ; b, cell with two colourless prolongations ; 

 c, with three ; d, with four ; and e, with only one 

 such prolongation. 



c. Siliceous concretions from the test of Salpa maxi- 

 ma, magnified 350 times. (After Lowig and Kolliker). 



D. Calcareous concretion from, test of Leptoclinium 

 maculosum. (After Milne-Edwards.) 



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 



