1198 



TUN1CATA. 



The common test of Aplidium gibbulosum 

 presents a homogeneous substance with some 

 scattered nuclei, and a great quantity of round 

 cells, with very delicate membranes, measur- 

 ing 0-005'" to 0-0 13'", and even to 0-02'". 

 Those in the interior contain only a liquid ; 

 but the nearer they approach the exterior, the 

 more are they found to contain calcareous 

 concretions ; and, lastly, there are cells per- 

 fectly incrusted, but without appendages. 

 Acted upon by soda and by hydrochloric acid, 

 the homogeneous substance alone remains, 

 the rest is dissolved. 



In the " common body " of Botryllus vio- 

 laceus are some remarkably incrusted cells, 

 similar in some respects to those of Didemnum. 

 Some of these are perfectly round, with a 

 diameter of O'OOO"', and, as those of Aplidium t 

 are filled with calcareous concretions ; others, 

 for the most part pyriform or fusiform, have 

 one or two pointed, colourless prolongations, 

 O'OOG'" 0-009'" in length, and are organic 

 in substance ; others, lastly, round or tetra- 

 hedral, have even three or four of these 

 prolongations, which are often of a similar 

 length, and are regularly disposed, but just as 

 often are of different sizes, and without sym- 

 metry (fig.llk. B).* These prolonged cells 

 are probably analogous to the round cells in 

 Didemnum, that have lime in their interior and 

 calcareous deposits externally, and may even 

 be compared with vegetable cells (pollen- 

 granules, spores, &c.), bearing external de- 

 posits. Were these cells incrusted, they would 

 form star-like bodies, similar to those of the 

 Didemnum. 



Professor M. Edwards observes, that in Lep- 

 toclinum the substance of the tissue is crowded 

 with calcareous granules, which, seen with an 

 ordinary lens, appear to be little spherical con- 

 cretions, but which are aggregations of little 

 pyramidal crystals, united by their base, so as 

 to represent a many-rayed star, surmounted 

 on each of its faces by a group of other 

 similar, but smaller, rays (see Jig. 774. D). 



The structure of the common test of the 

 Botryllus polycyclus is peculiar and quite dif- 

 ferent from those described above. In the 

 exterior parts of the common mass the struc- 

 ture resembles that ordinarily found in com- 

 pound Ascidians, being of a clear and homo- 

 geneous substance, with some nuclei and crys- 

 tals ; but in the interior distinct fibres are 

 found, by the side of the nuclei. These fibres 

 are of two kinds ; some, the least numerous, 

 are long, extremely pale and delicate, too fine 

 to be measured, and, crossing one another in all 

 directions, form elegant sinuosities ; others, less 

 numerous, are short, O'O I/" toO'OS^in length, 

 larger, opaque, and variously curved ; in a 

 word, they resemble certain nuclei transformed 

 into fibres (Kernfasern). Like the homoge- 

 neous substance of the exterior and interior 

 parts, these fibres resist the action of hydro- 

 chloric acid and of soda ; and consequently, 



* Specimens of these spicula in Mr. Bowerbank's 

 tistological collection exhibit much more coarsely 

 granulated centres than those here figured. 



since they are incontcstably organic, they are 

 composed of non-azotised substance. 



There are also some round points, visible 

 to the naked eye, dispersed in the common in- 

 tegument of this Botryllus. Some are white, 

 generally situated towards the interior of the 

 test, and appear under the microscope as 

 groups of granules or spiculae ; they are in- 

 soluble in a solution of soda, or in hydro- 

 chloric acid, and are probably siliceous, like 

 the concretions in Salpa. The other spots are 

 violet-coloured, or reddish, and are most 

 abundant in the external layers near the 

 groups of individuals, yet sometimes also 

 they are present in the interior parts of the 

 common mass. Seen under the microscope, 

 they appear as pyriform, round, or elongated 

 vesicles, bearing a reddish colouring matter, 

 contained probably in the cells, and are at- 

 tached to the extremities of the ramifying 

 canals that traverse the mass in every direc- 

 tion. These vesicles are the germs of new 

 individuals.* 



The coriaceous test of the Cynthia (Fam. 

 Ascidiadce) presents a composition still more 

 remarkable than that of the Bolrylli. In Cynthia 

 papillata, the fibres that constitute a large 

 proportion of the test, are in some parts so 

 much developed, that they may bear compari- 

 son with the fibrils of any fibrous tissue found 

 in vertebrate animals. 



In examining the structure of the test, where 

 it is gf no great thickness, we find a simple 

 lamina, quite on the interior surface, consist- 

 ing of an epithelium, with polygonal cells, which 

 is united to the mantle by scattered, crossing, 

 muscular fibres. A thick layer of fibres suc- 

 ceeds, having cells and nuclei disseminated in 

 it. The fibres themselves are colourless, un- 

 dulating, resembling the fibrils of the fibrous 

 tissue of vertebrate animals, but narrower ; 

 they measure 0-0002'" 0-0004"' in thick- 

 ness; they are never ramified, nor united into 

 bundles. In their direction they are in part 

 parallel to the axis of the animal, as in the 

 interior laminae ; and in part differently in- 

 terlaced, so that some are disposed longi- 

 tudinally, and others transversely (circular 

 and longitudinal fibres) ; hence, from the 

 disposition of the two kinds of layers, the 

 test can very easily be split into, sometimes 

 very delicate, laminae. There is apparently no 

 intermediate substance accompanying the 

 fibres, but the lamina? formed by their diver- 

 gences are occupied by a quantity of granules 

 and vesicles of different forms. Firstly, there 

 are minute colourless molecules, in some 

 parts so abundant as to render the fibres in- 

 distinct, and to give to some of the thicker 

 layers a finely granular aspect; secondly, 

 crystals, which are present only in the ex- 

 terior layers ; thirdly, nuclei, measuring 

 O'OOl'" to 0-003'", often with large granules, 

 apparently of fat ; fourthly, cells of different 

 forms. Some of these cells, containing nuclei 



* The epiderm covering the common gelatinous 

 test of Botryllus stdlatus is very tough, closely ad- 

 herent, and presents distinct cellular structure. 



