The fibrous tissue of the Cynthia ponzaria 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 yellina, Diazona, Botryllus, Pyrosoma, and Saa ; 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.
" of MM. Liiwig and Kolliker was examined by a committee of the French Institute, consisting of D umas,Milne-Edwards, 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: — Illy Ulf "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 L6wig 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 lusiT, Clavellince, and Diazonce, 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 Aravicula, Frustulia, Baccilaria, Closterium, &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 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. Liiwig and 'Whiter 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 Aplidium, 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.