HUXLEY, ON APPENDICULAKIA FLABELLUM. 183 



account of tlio matter, trusting that further researches may clear up the 

 point. 



" The formation of the envelope or ' Haus' commences by the develop- 

 ment of a lamina from the ' semicylindrical organ' (ganglion?). This, as 

 it grows, protrudes through the opening at the apex of the animal (respi- 

 ratory aperture). Its corners then become bent backwards and inwards, 

 and thus a sort of horn is formed on each side, the small end of which is 

 turned towards the apex of the animal, while its mouth looks backwards, 

 downwards and outwards. 



"At the same time two other horns are developed upwrirds (the animal 

 is supposed to have its small end downwards), one on each side. These 

 are smaller and more convoluted than the others. 



" This four-horned structure consists of a very regular network of 

 vessels, in which, at the time of the development of the organ, a very 

 evident circulation is visible ; the blood-corpuscles streaming from the 

 attached end of the organ. ' The clearness with which the circulation was 

 perceptible, together with the great abundance of vessels and the large 

 extent over which they were spread, were circumstances which led me 

 (says Mertens) to believe this truly enigmatical structure to be an organ, 

 whose function was the decarbonization of the blood. The ease with wliich 

 the animal becomes separated from this organ is no objection to this view ; 

 the necessity there seems to exist for the reproduction of the latter rather 

 confirming my opinion.' 



" It is highly desirable that more information should bo gained about 

 this extraordinary respiratory organ, which, if it exist, Avill not only be 

 quite sui generis in its class, bat in all animated nature. And in a phy- 

 siological point of view, the development of a vascular network, many 

 times larger than the animal from which it proceeds, in the course of half 

 an hour, will be a fact equally unique and startling." 



" For my own part, I think there can be no doubt that the animal is 

 one of the Tunicata. The whole organization of the creature, its wide 

 respiratory sac, its nervous system, its endostyle, all lead to this view. 



" In two circumstances, however, it differs widely from Tunicata 

 hitherto known. The first of them is, that there is only one aperture, the 

 respiratory, the anus opening on the dorsum ; and secondly that there is a 

 long caudal appendage. 



" As to the first difference, it may be observed, that, in the genus 

 Pelonaia* an undoubted Ascidian, there are indeed two apertures, but 

 there is no separation into respiratory and cloacal chambers. Suppose 

 that in Felonaia the cloacal aperture ceased to exist, and that the rectum, 

 instead of bonding down to the ventral side of the animal, continued in its 

 first direction and opened externally, we should have such an arrangement 

 as exists in Api^endicularia. 



" With regard to the second difference, I would remark, that it is just the 

 existence of this caudal appendage which makes this form so exceedingly 

 interesting. 



" It has been long known that all the Ascidians commence their ex- 

 istence as larvae, swimming freely by the aid of a long caudate appendage ; 

 and as in all great natural groups some forms are found which typify, in 

 their adult condition, the larval state of the higher forms of the group, so 



* I would particularly remark that the statement that there is no separation 

 between the branchial and cloacal chambers in Felonaia is erroneous. At the time 

 this paper was written I had not examined Pelonaia (whose structure, as I have since 

 found, differs in no essential point from that of an ordinary Cynthia), and I must have 

 misunderstood the verbal infonnation given by my lamented friend Professor E. 

 Forbes. 



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