ANATOMY AND PHYSIOLOGY. 55 



formed; and then another series of secondary vessels is 

 developed,, and afterwards another primary vessel, and so on, 

 gradually increasing the length of the two branchial leaflets 

 (if they may be so called), which at the same time grow in 

 breadth, passing further and further across the thoracic cavity 

 until at length they reach the sides of the endostyle ; all the 

 Avhile the primary and secondary vessels along the margins of 

 the growing organ, open into the pallial sinuses in the manner 

 already indicated ; so that the boundary of the water-space or 

 atrium is well defined, and is always 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 ; 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 un- 

 doubtedly originates 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 connected, 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 mar- 

 gin 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, neces- 

 sitated 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 each side of this 

 line, the great ventral blood-channel apparently not being 

 yet detached from the inner tunic. It is not till the " branchial 

 sac is everywhere perforated " that these water-spaces, accord- 

 ing to this naturalist, are united by the formation of the 

 cloaca. I have certainly not seen the young of Clavetina in a 

 sufficiently early stage of development to warrant the asser- 

 tion that such does not take place in this form ; but assuredly 



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

 Memoirs/ edited by Henfrey and Huxley, 1853, p. 324. 



