HUXLEY, ON APPENDICULARIA FLABELLUM. 185 



genus, A, furcata, A., acrocerca, A. cophocerca, and A. cceru- 

 lescens. The most important and novel point in Dr. Gegen- 

 baur's paper, hoAvever, is the discovery and description of the 

 true branchial apertures, which had been overlooked by all 

 previous observers, Dr. Leuckart and myself included. Dr. 

 Gegenbaur says (1. c, p. 415) — 



" If now we return to the branchial sac, the most remarkable objects are 

 the two respiratory clefts which lie on its ventral wall and partially em- 

 brace the entrance into the o?sophagus. Hitherto, none of those who have 

 investigated the Aj)pendiculari(ji have recognised the true import of these 

 organs, although Mertens saw them in Oikopleura Cliamissonis, and 

 Busch (in Euryccrtis pallidus) would, in all probability, have understood 

 them had he only borne in mind the typical structure of the Ascidians. 

 Neither Huxley nor Leuckart have mentioned these respiratory aper- 

 tures." 



After describing the apertures, Gegenbaur proceeds — 



" Exact observation shows that they are not simple apertures in th® 

 branchial sac like those of the Ascidians, connecting its cavity with i\fi 

 surrounding space ; but that each is prolonged into a short tube which 

 converges more or less towards its fellow on the ventral face." 



In A. furcata these two tubes run 



" At first parallel with one another downwards (if the animal be sup- 

 posed to have its anterior part directed upwards, as in the figures), then 

 form a knee-like curve inwards, running directly towards one another, 

 and then entirely vanish, so that nothing more could be made out as to 

 their mode of termination. The function of the respiratory apertures is 

 therefore, in this case, entirely different from that which they perform iu 

 the Ascidians, in which the water j^asses through the branchial clefts, 

 and, after aerating the blood contained in the network of the branchial 

 vessels, collects in the space between the mantle and the branchial 

 sac, to be eventually poured out of the cloacal aperture ; while in our 

 AiypendicidariiK the water is led back by tubular prolongations of the 

 branchial clefts into the body, so as either to become directly mixed witli 

 the blood, or by some further ramifications of the tubes to act through 

 their thin walls on the surrounding blood. Which of these possibilities 

 really occurs must remain, for the present, undecided ; for although in 

 A. cophocerca the end of the respiratory tube may be seen very clearly, 

 yet it is still uncertain whether a bent prolongation of it may not 

 be continued from this point, and may not, by presenting a transverse 

 sectional view, give rise to the appearance of an end. I will enter no 

 further in this place into the discussion of possibilities, my principal 

 object being the statement of facts. However, I believe I have demon- 

 strated that there is a tolerably-marked difference between the respiratory 

 system of the Ascidians and that of the Appendicidarioe, expressed mor- 

 phologically by the tube proceeding from the respiratory apertures of the 

 latter." 



Excessively puzzled to understand how structures so well 

 marked and so obvious as these should have escaped my 

 notice, I was, as may be imagined, very desirous to re-examine 

 Appendicularia ; but although its occurrence in the British 



