46 THE VOYAGE OF H.M.S. CHALLENGER. 



the peribranchial cavity, and forming the "lining membrane" of Hancock (the "third 

 tunic" of Milne-Edwards) (see fig. 9, page 40). 



The typical arrangement among Simple Ascidians (e.g., Ascidia mentula) is for the 

 oesophagus to run posteriorly and ventrally from the branchial sac, to open into the 

 large fusiform, globular, or cylindrical stomach, which extends towards the ventral edge 

 of the mantle cavity (fig. 12). The intestine emerges from the most ventral and 

 anterior part of the stomach, and runs anteriorly for a short distance, and then turns 

 dorsally and posteriorly, so as to point towards the stomach. Before reaching that organ, 

 however, it bends dorsally and then anteriorly, becoming the rectum, which runs forwards 

 near the dorsal edge of the peribranchial cavity to terminate in the anus, usually situated 

 near the atrial aperture (fig. 12, a, and fig. 1, page 27). 



This arrangement prevails with slight modifications throughout the genera Ascidia 

 and Pachychlcena. In Ciona — a simpler form — the stomach and part of the intestine 

 lie posteriorly to the branchial sac, and the rest of the intestine has a straighter 

 course than is the case in Ascidia. In the genus Corella the alimentary capal lies at 

 the right side of the branchial sac, and the intestine turns posteriorly on leaving the 

 stomach. After a very short course it bends dorsally, and after running parallel to 

 the stomach and oesophagus it turns anteriorly, and becomes the rectum. Consequently 

 the three genera — Ascidia, Ciona, and Corella — have the relations of the branchial sac and 

 the intestine very different. Ciona seems to be the more primitive form, from which 

 the other two may have been derived. 1 In Ahyssascidia the arrangement is much the 

 same as in Corella, but the entire canal is more closely packed at the posterior end of the 

 body (PL XXVII. fig. 3). 



In most of the Clavelinidse the arrangement resembles that found in Ciona. In the 

 genera Clavclina, Ectcinascidia, and Rhopalwa, the intestine is greatly prolonged pos- 

 teriorly to the branchial sac, so as to form a fairly distinct region of the body, named by 

 Savigny the abdomen. In Perophora, on the other hand, there is no distinct abdomen, 

 the intestine having become drawn up alongside the branchial sac on its left side. 



In the Cynthiidaa there is considerable variation in the situation, length, and 

 disposition of the intestine. Probably the simplest condition is that found in Culeolus 

 among the Bolteninse. Here the atrial aperture has retained what is probably its primi- 

 tive position, at or near the posterior end of the body (PI. VIII. fig. 8), and conse- 

 quently the anus opens in that region of the peribranchial cavity, thus dispensing with 

 the long anteriorly running rectum so well developed in Ascidia. Consequently the 

 intestine, after leaving the stomach, merely turns forwards and then backwards again 

 towards the atrial aperture. 



In the Molgulidse the stomach and intestine lie upon the left side of the branchial sac, 

 and form a single, transversely directed, narrow loop, the posterior segment of which is 



1 See Herdman, Notes on British Tunicata, Jour. Linn. Soc, Zool., vol. xv. p. 274, 18S0. 



