TEGUMENTARY ORGANS 407 



can be effected with considerable difficulty, or not at all, and this 

 difference has even been raised to the rank of a zoological distinction, 

 the Ascidians having been in consequence divided into Monochitonida 

 and Dichitonida. I believe, however, that in all those Ascidians 

 whose test is unprovided with vessels, it is, normally, closely adherent 

 to the outer tunic, and I am inclined to think that this is equally true 

 even of those forms, such as the Phallusise, in which in preserved 

 specimens the test and outer tunic are so commonly found detached 

 from one another. Here, however, as the test is provided with an 

 abundant vascular supply proceeding from one point of the body, it 

 may normally become separated elsewhere. Careful examination of 

 fresh specimens can alone decide this point. 



The simplest form of Ascidian test is that presented by the Salpse. 

 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 surrounded 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 structure which, on the one 

 hand, resembles bone, on the other some forms of fibro-cartilage. The 

 endoplasts, in fact, have become surrounded by cell walls, which are 

 produced into long, frequently anastomosing processes {fig. 314. D) ; 

 these retain their animal composition, while all the immediate tissue 

 is strongly impregnated with cellulose. This is the fundamental 

 structure of the test in the Phallusise and Clavelinse also ; but here an 

 additional complication results from the development in the substance 

 of the test of a series of rounded cavities, which gradually enlarge 

 until they almost come into contact, and give rise to a spongy texture. 

 The intervening septa at the same time frequently become obscurely 

 fibrous {fig. 314. c). Now these " vacuolse," whose origin and nature 

 appear to me to show their identity with the " cancelli " of bone 

 developed from cartilage, have been described by Lowig 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 vacuolse, but may with care be demonstrated to exist up to 



their very edges. 



Botryllus, Synoicum, Syntethys, Boltenia, and the Cynthise, 

 present a new series of appearances : 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. 



