ANATOMY AND PHYSIOLOGY. 23 



THE DIGESTIVE SYSTEM. 



The most interesting matter that I have to communicate re- 

 specting the digestive system relates to the biliary apparatus. 

 A remark or two, however, may be made, in the first place, 

 upon the alimentary canal, which, in all the species tha-t have 

 come under my inspection, makes its first bend towards the 

 dorsal region, assuming that to be the dorsal aspect where the 

 endostyle is placed. The intestine then usually ascends and 

 crosses over (in a more or less nndulatory course, sometimes 

 forming one or two loops) to the opposite or ventral side, 

 where it again ascends to reach the cloaca, into which, in the 

 Ascidians, it invariably opens. The walls, from one end of 

 the organ to the other, are particularly firm, and do not col- 

 lapse even in preserved specimens. The lower portion of the 

 intestine is the most delicate ; but even here the wall rarely 

 shrinks. The stomach is well marked, though it is never very 

 bulky, and is usually lined with a stout mucous membrane, 

 which is frequently plaited or wrinkled, sometimes in a sym- 

 metrical iiianner, the plaits extending into the oesophagus on 

 the one hand, and into the intestine on the other. In the 

 latter organ this membrane is thrown up so as to form a very 

 conspicuous groove which extends from the stomach to that 

 portion of the intestine which may be termed the rectum. In 

 Xtt/ela t'uberosa, and some other species, however, this groove 

 extends the whole length of the intestine. 



The food of the Tunicaries is extracted from sedimentary 

 matters ; there is no power of selection in the first instance ; 

 those particles which can be, are digested ; the others, chiefly 

 composed of sand and mud, are rejected in the usual manner. 

 The sedimentary aliment is sifted from the water in the respi- 

 ratory sac by the aid of the branchial network, and is then 

 carried across the organ by the action of cilia; but no definite 

 arrangement of the particles takes place until they arrive at 

 the oral or ventral lamina, where they are formed into a cord 

 of some tenacity, apparently through the agency of mucus, and 

 are carried thus moulded along this lamina to the oral orifice, 

 and so swallowed. This alimentary cord is conducted through 

 the digestive tube, and is rejected in the same form by the 

 anus and excurrent tube. The cord-like fasces may frequently 

 be seen through the wall in the lower portion of the intestine, 

 having very much the appearance of a convoluted tube lying 

 within the canal. In some of the lower forms, however, it is 

 broken up into elongated pellets. 



