100 BRITISH TUNICATA. 



considerably in advance of the level of the mouth, 

 terminates in a wide anal orifice bordered with a 

 broad, reflected, lobed, margin. The looped portion 

 of the intestine is wider than the rectal portion, and its 

 walls are delicate and membranous. A thick mucous 

 membrane lines the whole alimentary tube. In the 

 stomach it is thrown into rugae which can be observed 

 from the outside as narrow, opaque, yellowish lines 

 sloping forwards and outwards from the centre of the 

 organ. The lining membrane of the intestine is raised 

 into a strong ridge which extends the whole length of 

 the tube, and is seen through the left wall as a broad 

 dark groove-like line. 



The liver in Sfyela is not more conspicuous than it 

 is in Aseidia; and it shows about the same degree of 

 development, being provided with its secreting vesicles 

 and ducts, only slightly modified from those of that 

 genus. In S. tuberosa, and indeed in all the members 

 of this group which have come under our observation, 

 there is a fold of the lining membrane within the loop 

 of the alimentary tube, which passes between the 

 stomach and intestine. This fold is united to the 

 pyloric end of the stomach, where there is a ca3cal pro- 

 longation of that organ. The hepatic ducts lie within 

 this fold, and before they reach the stomach, in this 

 species, the^y unite to form a simple, slender duct, which 

 opens into the left side of the cascum. The branches of 

 the ducts ramify dichotomously over the looped portion 

 of the intestine, and communicate with comparatively- 

 large, round vesicles arranged like those of Peloitaia. 



The alimentary tube in S. mamillaris differs from 

 that of S. tnlerosa in some unimportant particulars. 

 The oesophagus is much longer, and the stomach is 

 placed transversely almost at the bottom of the pallial 

 chamber. The intestine runs backwards and nearly 

 extends as far as the dorsal border of the chamber 

 before it ascends to form a wide transverse loop ; it 

 then crosses to the cloaca, making a deep sigmoid 

 curve, and terminates at a little distance above the 



