340 MR. A. HANCOCK ON THE ANATOMY 



coextensive with the expanding gill. The oral processes, which 

 in this animal occupy the place of the lamina, are produced one 

 by one, in accordance with the appearance of the primary vessels 5 

 and the vascular suspenders likewise originate at the same time. 



It is unnecessary on this occasion to trace the development of 

 the branchial sac further, or with more minute details ; it should 

 be observed, however, that the growth of the gill undoubtedly ori- 

 ginates in the great ventral channel, which is itself a production 

 of the lining membrane, and that during the development of the 

 organ it is connected with this membrane, and that this connexion 

 is ever afterwards maintained by the vascular suspenders. It may 

 also be remarked that in no stage of the growth is the gill ever con- 

 nected, on the one hand, with the margin of the oral orifice — or, on 

 the other, with the tentacular filaments of the incurrent tube, which 

 are, indeed, placed at a considerable distance from the upper margin 

 of the gill ; and the lower margin is some way above the oral orifice. 



The above description of the development of the gill does not 

 exactly agree with that given by A. Krohn of the branchial sac of 

 Phallusia (Ascidia) mamillata*. According to this author, there 

 are at a very early stage of development two excurrent orifices, 

 one on each side of the middle line, — necessitated by the fact that 

 the gill commences to separate itself from the walls of the cavity 

 at two points simultaneously, thus forming two separate water- 

 spaces, one on eachside of this line, — thegreatventralblood-channel 

 apparently not being yet detached from the inner tunic. It is 

 not till the " branchial sac is everywhere perforated" that these 

 water-spaces, according to this naturalist, are united by the for- 

 mation of the cloaca. I have certainly not seen the young of 

 Clavelina in a sufficiently early stage of development to warrant 

 the assertion that such does not take place in this form; but 

 assuredly in it, at a very early period, the cloaca freely communi- 

 cates with the water-space or atrium on each side. At the same 

 time it must be allowed that it is more than probable that, at the 

 earliest stage of existence, in Clavelina and in other Tunicates the 

 great ventral channel is united throughout to the wall of the pal- 

 lial chamber ; and hence the statement of Krohn does not seem 

 at all unlikely. And, moreover, we thus learn that this great 

 blood-channel is developed in connexion with the lining membrane, 

 with which it continues ever afterwards more or less connected. 

 We have already seen that the transverse or primary vessels take 

 their origin in this same vessel, and that they in their turn give 



* "On the Development of the Ascidians," by A. Krohn — 'Scientific Me- 

 moirs', edited by Henfrey and Huxley, 1853, p. .'524. 



