194 AMPHIOXUS. 



of the pharynx, which is folded longitudinally in 

 its hinder part to form a groove. The endostyle 

 also contains mucous glands. 



4. The intestine commences at the hinder end of the 

 pharynx, close to the dorsal surface, and runs 

 straight to the anus. It is a direct continuation of 

 the epibranchial groove, and at its commencement 

 is very narrow ; further back it dilates consider- 

 ably ; and this expanded part, which is sometimes 

 called 'stomach,' is produced into a large pouch- 

 like sac the liver which runs forwards some 

 distance along the right side of the pharynx, and 

 ends blindly in front. 



E. The Atrial Cavity. 



The boundaries and relations of this cavity can best be 

 determined by the examination of a series of transverse 

 sections. 



The atrial or epipleural cavity is the large place which 

 surrounds the pharynx at the sides and below : into it the 

 stream of water passes from the pharynx through the gill- 

 slits, to escape finally from the atrial pore. 



The atrial cavity develops in the embryo as a groove 

 along the under surface of the body ; by fusion of its lips 

 the groove becomes a tube, which ends blindly in front, but 

 retains its opening behind at the atrial pore. The cavity 

 extends laterally so as to surround the pharynx both below 

 and at the sides, lying between the pharynx and the muscular 

 body-wall. 



The epithelium lining the atrial cavity is supported by a 

 layer of connective tissue, and usually contains a good deal 

 of brown pigment, which renders it easy to trace. It is repre- 

 sented by the thick black line in figs. 43-46, which show its 

 distribution in different regions of the body. It lines the 

 inner surface of the atrial folds, covers the outer surface of 

 each of the gill-arches, and invests the liver and the anterior 

 portion of the intestine. 



