THE TUNICATA. 517 



way as the branchial chamber was laid open from the oral ap- 

 erture. The atrial opening is thus seen to lead into a cavity, 

 interposed between the branchial sac and the parietes and 

 lined upon all sides by a delicate membrane (the third tunic 

 of Milne-Edwards) like a peritoneum. This membrane has a 

 parietal and a visceral layer. The former is continued from 

 the atrial aperture on to the parietes of the body to the level 

 of the peripharyngeal band in one direction, to a line parallel 

 with the endostyle in another, and to the alimentary and 

 genital viscera in a third direction. From these various lines 

 it is reflected on the branchial sac, of which it forms the outer 

 wall. At the margins of the stigmata it is continuous with 

 the endoderm of the pharynx, and, at the aperture of the rec- 

 tum, with the endoderm of the intestine. Thus the atrial 

 membrane forms a bilobed sac, one lobe extending on each 

 side of the pharynx, and opens outward by the atrial aper- 

 ture ; it communicates by the stigmata with the interior of 

 the branchial sac, and, by the anal and genital openings, it 

 receives the faeces and genital products. The current which 

 sets in at the oral and out at the atrial aperture is set in mo- 

 tion by the cilia of the stigmata. 



The atrium of the higher Ascidians differs from that of 

 Appendicular ia, not only in extent, but in being single and 

 not double ; and in its single aperture being placed upon the 

 neural aspect of the body close to the ganglion, while the 

 atrial funnels of Appendicularia open upon the haemal aspect 

 of the body. The development of the higher Tunicata, how- 

 ever, shows that the peculiarities of the atrium in them are 

 of secondary origin ; and that, to begin with, there are two 

 distinct atria, as in Appendicularia. 



The oesophageal aperture is usually surrounded by a raised 

 lip, and the short and wide oesophagus leads into a dilated 

 stomach, whence a shorter or longer intestine proceeds. The 

 alimentary canal is always bent upon itself in such a manner 

 that the anus terminates on the neural side of the body, in 

 the atrial chamber. 



In Clavelina, Amouroucium, Didemnum, Syntethys, and 

 most of the compound Ascidians, the greater part of the ali- 

 mentary canal lies altogether beyond the branchial sac, in a 

 backward prolongation of the body which has been termed 

 the abdomen, and is often longer than all the rest of the 

 body ; the alimentary canal forming a long loop, and the di- 

 rection of the axis of the branchial sac being continued by 

 that of the gullet, stomach, and first half of the intestine. In 



