1200 



TUNICATA. 



or by pale granules, enveloped with an elegant 

 skein of fine, cylindrical, opaque fibres, which 

 can be isolated by compression. Whether the 

 fibres have a spiral arrangement is uncertain. 

 MM. Lowig and Kolliker, who discovered these 

 remarkable fibre-coated cells, remark that, as 

 to the manner in which this curious trans- 

 formation of the pigment-cells is brought 

 about, the increase of thickness in the cell- 

 membrane would take place by growth, or by 

 a development similar to that which occurs 

 in many vegetable cells, the membrane of 

 which consists of several layers, or by the de- 

 position of a substance applied externally. 

 As there is no appearance of any internal or 

 external deposit, and as the cell-cavity is not 

 diminished during the thickening of its walls, 

 they consider the first to be the more likely 

 cause of the above conditions, but, neverthe- 

 less, not sufficient alone to account for the 

 circumstances. It is difficult, perhaps, to say 

 how these fibres are formed, but it may be 

 that the cell-walls, by partial solidification, 

 ultimately become separable into fibres. 



The fibrous tissue of the Cynthia pomaria is 

 insoluble in hydrochloric acid, or in solution 

 of soda ; the other elements of the test, sub- 

 mitted to these agents, disappear. 



The following clever abstract of the facts 

 connected with the subject has been drawn up 

 by Professor Edward Forbes : 



" MM. Lowig and Kolliker found cellulose 

 undoubtedly present in the envelopes of many 

 Tunicata, both simple and compound, in- 

 cluding the genera Phallusia, Cynthia, Cla- 

 vellina, Diazona, Botryllus, Pyrosoma, and 

 Sal-pa ; but they sought in vain for cellulose 

 in animals of inferior organisation, although 

 in some of the above-named creatures it 

 formed a very considerable part of the animal 

 tissues. 



" The Memoir of MM. Lowig and Kolliker 

 was examined by a committee of the French 

 Institute, consistingofDumas,Milne-Ed wards, 

 Boussingault, and Payen ; the last-named 

 eminent philosopher drew up the report. In 

 it he gives the following formula of the com- 

 position of the envelopes of the Tunicata : 



Cellulose 



Azotised substance 

 Inorganic matter - 



60-34< 

 27-00 

 12-66 



lOO'OO 



" He remarks that the establishment of 

 the existence of cellulose in the Tunicata is 

 a 'fait capital' in science, very important in 

 its bearing on future researches into the com- 

 parative physiology of the two kingdoms. 



" The explanation offered by Lowig and 

 Kolliker of these very anomalous facts is ex- 

 tremely ingenious, and probably very near the 

 truth. It is to the following effect: Tuni- 

 cata live entirely upon vegetable organisms. 

 The contents of the stomachs of the Phal- 

 lusia;, ClavcllinfE, and Diazona;, examined, con- 

 sisted of particles of florideous algae, which 

 had probably found their way there by chance, 

 and a great quantity of microscopic plants of 

 low position in the series, species of Navicula, 



Frustulia, Baccilaria, Clostcrium, &c. These 

 minute vegetable organisms have been shown 

 by Nageli and Schmidt to contain cellulose. 



" This is probably dissolved by the gastric 

 juice, that is to say, changed into sugar or 

 gum, in which state it circulates with the 

 blood, and is afterwards introduced into the 

 tunics, either directly by the sanguiferous 

 canals (as in Phallusia), or by their prolonga- 

 tions ramified in the walls of the common 

 body (as in Diazona and Botryllus), which 

 thus, as Milne Edwards has shown, contain 

 also blood in their cavity, probably pene- 

 trating by imbibition when the envelopes have 

 no bloodvessels. The presence of cellulose 

 in the tunics of the ascidian Molluscs, then, 

 cannot be taken as an evidence of an ap- 

 proach to a vegetable nature in those bodies. 

 It affords us, however, a wholesome warning 

 against the placing of confidence in asserted 

 chemical distinctions between the great king- 

 doms of Nature." * 



From the observations made by MM. Lowig 

 and Kolliker on the histological characters of 

 the embryos of certain compound Ascidians, 

 they arrived at two important results. Firstly, 

 that the external structureless envelope of the 

 embryos, which, apparently, is identical with the 

 external envelope of the adults, is, according 

 to their analysis, composed of cellulose ; that 

 this envelope is formed only when the division 

 of the yolk is accomplished, and even when 

 the exterior form of the embryo is indicated. 

 Secondly, that this envelope, subsequently 

 containing, as in Botryllus and in Aptidium, 

 another structureless substance, fibres, nuclei, 

 and crystals, is primitively altogether homo- 

 geneous and unorganised. 



Hence it appears that the test of the Tuni- 

 cates is a product of the activity of cells formed 

 subsequently to the process of the division of 

 the vitellus, and that primitively it is only a 

 mass secreted by these cells. Its ulterior 

 organisation is not yet understood, but remains 

 an open field for much interesting and im- 

 portant research. 



ANATOMY OF THE ASCIDIAD^E. We have 

 already referred to the external envelope of 

 the Tunicates as being analogous to the cal- 

 ciferous shells ortests of the other Acephalans; 

 and the muscular sac enclosed within it, as the 

 analogue of the mantle; and the membranous 

 sac lining the mantle as analogous to the 

 branchiae of the Acephalans. f 



The rest of the viscera of the Ascidians are 

 enveloped in a peritoneum J, and the heart has, 

 besides, its own membranous sac or pericar- 

 dium. Thus the body, properly so called, 

 appears as if divided into three cavities ; that 

 of the branchiae, communicating with the 

 exterior by the superior opening of the sac, 



* Hist. Brit. Molluscs, vol. i. p. 6. 



t According to Van Beneden, the longitudinal 

 vessels of the branchije of the Ascidia; are more par- 

 ticularly the analogues of the ciliated tentacles of 

 the Bryozoa. 



J The peritoneum of Ascidia intestinalis offers a 

 good example, being of firm consistence, circum- 

 scribing and protecting the abdomen. 



