494 



TEGUMENTARY ORGANS. 



fal examination of fresh specimens can alone 

 decide this point. 



Fig. 314. 



The simplest form of Ascidian test is that 

 presented by the Salpae. Here, we have 

 merely a gradual growth of the periplast and 

 a deposit of cellulose within it, the endoplasts 

 either remaining as such or becoming sur- 

 rounded by cell walls. The resulting tissue, in 

 fact, is identical with cartilage, if we suppose 

 cellulose to have taken the place of chondrin. 



In the Pyrosomata, the test has a struc- 

 ture which, on the one hand, resembles 

 bone, on the other some forms of fibre-carti- 

 lage. The endoplasts, in fact, have become 

 surrounded by cell walls, which are produced 

 into long, frequently anastomosing processes 

 (./%. 314. D) ; these retain their animal com- 

 position, while all the immediate tissue is 

 strongly impregnated with cellulose. This 

 is the fundamental structure of the test 

 in the Phailusiae and Clavelinas also ; but 

 here an additional complication results from 

 the development in the substance of the test 



of a series of rounded cavities, which gradu- 

 ally enlarge until they almost come into con- 

 tact, and give rise to a spongy texture. The 

 intervening septa at the same time frequently 

 become obscurely fibrous (Jig. 314. c). Now 

 these " vacuolse," whose origin and nature ap- 

 pear to me to show their identity with the 

 " cancelli " of bone developed from cartilage, 

 have been described b}' L6 wig and Kolliker, and 

 by Schacht as cells ; and the latter has even 

 stated that they possess a nitrogenous lining 

 membrane. This is, however, a mistake, 

 arising from the imperfect operation of the 

 reagents by which the cellulose is detected ; 

 it is simply less abundant close to the cavities 

 of the vacaolae, but may with care be demon- 

 strated to exist up to their very edges. 



Botryllus, Synoicum, Syntethys, Boltenia, 

 and the Cynthias, present a new series of ap- 

 pearances : here the periplast of the ecderon 

 is metamorphosed into fibres, which, however, 

 are not composed of pure cellulose, but of a 

 nitrogenous substance impregnated therewith. 

 In Synoicum the test is soft, and presents very 

 much the structure of some forms of rudimen- 

 tary connective tissue. We find, in fact, a 

 more or less distinctly fibrillated basis with 

 scattered endoplasts ; some of these are in- 

 vested by round granulous nitrogenous cell- 

 walls, while in others the cells are spindle- 

 shaped and prolonged at each end into fibres 

 (representing thus the elastic element of or- 

 dinary connective tissue), or they may be 

 stellate. Botryllus, Syntethys, and Boltenia, 

 present a similar structure, varying, however, 

 in the extent to which the nitrogenous cell- 

 walls on the one hand, and the periplast im- 

 pregnated with cellulose on the other, have 

 undergone development. Thus the periplast 

 is broken up into very obvious fibres in Bo- 

 tryllus, while in Boltenia the fibrillation is 

 pale and indistinct. On the other hand, I 

 have nowhere met with so great a develop- 

 ment of the nitrogenous cell-wall as in 

 Synoicum. 



In Boltenia a more or less distinct lamina- 

 tion makes its appearance in the test, and this 

 peculiarity, as well as the fibrous structure 

 altogether, attains its maximum in the Cyn- 

 thiae. In Cynthia papillata, for instance, the 

 middle substance of the test is composed of 

 numerous, very obvious laminae, which con- 

 sist of fibres directed alternately parallel 

 with, and perpendicular to, the surface of 

 the test (314. B.) At first sight, they appear 

 as Lowig and Kolliker have described them, 

 to be decussating sets of longitudinal and 

 radiating ; but on a careful examination 

 of their sections I invariably found that the 

 apparently radiating fibres bend round as they 

 approach the apparently longitudinal set, and 

 in fact pass into the latter. The longitudinal 

 bands are, however, no thicker at one end of 

 a section than at the other, so that the trans- 

 verse fibrils cannot be merely given off from 

 them. A transverse section, again, exhibits 

 the same appearances as a longitudinal one ; 

 so that 1 think the fibres must in reality 

 have a more or le.xs regularly circular arrange- 



