326 A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 



the two siphons gradually approximate towards the middle line 

 of the dorsal surface and at last come so close, that they are 

 separated only by a small bridge. When the respiratory sac is 

 completely perforated, this also disappears. Instead of the pre- 

 vious two excretory siphons, we now find a single one situated 

 exactly in the middle of the dorsal surface, close behind the 

 nervous ganglion ; its lip is already, as in the adult animal, pro- 

 vided with six lobes. 



The presence of two excretory siphons, at an earlier period, 

 need not be at all surprising, if we recollect what has been pre- 

 viously stated respecting the two spaces which were formed over 

 the original gill-clefts ; it appears, in fact, to be almost a neces- 

 sary consequence. It was shown that the two spaces are deve- 

 loped by the raising up of the second layer of the body, in the 

 region of the apertures, from the respiratory sac, which is else- 

 where closely applied to it. This mutual separation goes on 

 to a greater and greater extent, during the further develop- 

 ment of new clefts in the respiratory sac. The consequence is, 

 that the boundaries of the two spaces extend further and further, 

 until at last, when the branchial sac is everywhere perforated, 

 they coalesce upon the dorsal side, and in this manner consti- 

 tute that inner space, between the branchial sac and the second 

 layer of the body, to which we have already referred in the adult 

 animal. When this has taken place there is no longer any need 

 of more than one siphon, and, in fact, the formation of the single 

 siphon is contemporaneous with that of the space in question. 



I pass now to an organ, which in all the Phallusice surrounds 

 the whole nutritive canal, from the mouth to the anus, and 

 has the appearance of a compact honey-yellow mass, dotted 

 over with chalky-white points. By the majority of zoologists, 

 among whom we find a great authority (V. Siebold, Vergleichende 

 Anatomie, p. 269), this structure has been regarded as a liver ; a 

 view which, to me, appears so much the more doubtful, as an 

 organ which will subsequently be described, and which has 

 hitherto escaped notice, probably has a greater right to this title. 

 The organ now in question consists of obvious, clear, round, 

 tolerably dense walled vesicles, the supposed simple glandular 

 utricles in which the bile is prepared. 



Every vesicle is apparently distended by a transparent fluid, 



