66 BRITISH TUNICATA. 



The moutli opens at the bottom of the branchial sac 

 towards the ventral margin. In some of the species, 

 however, as in A. mentnla, for example, the branchial 

 sac extends backwards some way behind the oral 



e/ 



orifice. The oesophagus is short and constricted ; it 

 opens into the anterior or ventral extremity of a well- 

 marked stomach, which is usually a wide rounded 

 sac laterally compressed, lying diagonally across the 

 pallia! chamber between the right pallia! wall, to which 

 it is adherent, and the branchial sac. The intestine 

 is a wide even tnbe. It passes from the opposite or 

 dorsal extremity of the stomach, and ascends for some 

 distance, when it crosses over to the other or ventral 

 side in a more or less nndnlatory course, thus forming 

 one or two loops, and then ascends again to reach the 

 cloaca a little in advance of where the oesophagus joins 

 the branchial sac. Here it terminates in a wide anal 

 opening. 



The whole alimentary tube is lined with a stout 

 mucous membrane which is frequently plaited or 

 wrinkled. This membrane forms in the intestine a 

 strongly denned groove which is extended into the 

 stomach. 



In all the As<-i<li;i' the alimentary tube is coated 

 externally with a pretty thick layer of a gland-like 

 substance composed of comparatively large, globular 

 vesicles with thin reticulated walls, each having in its 

 wall a large, opaque, simple or compound nucleus 

 (fig. 2, p. 24). These vesicles have no communica- 

 tion with each other, they are not connected with any 

 duct, nor do they open in any way into the alimentary 

 tube. This gland-like substance cannot subserve the 

 function of a liver, though by some naturalists it has 

 been suggested that it does so. 



The true hepatic organ lies beneath this vesicular 

 mass, and forms a thin coating on the surface of the 



tj 



intestine (fig. 3, p. 25). It is composed of delicate 

 tubes dividing dichotomously, but frequently without 

 much regularity. At the points where the branches 



