186 AMPHIOXUS. 



cells runs along the endostyle or mid-ventral wall 

 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, arid 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 considerably ; 

 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. 41-48, which shows its 

 distribution in different regions of the body. It lines the 

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



