ATEIAL CAVITY 177 



to the anus. It is a direct continuation of the hy- 

 perbranchial 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. 



D. The Atrial Cavity. 



The boicnda/ries and relations of this cavity can only be 

 determined by the examination of a series of transverse 

 sections. 



The atrial or epipleural cavity is the large space which 

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

 respiratory stream of water passes from the pharynx through 

 the gill-sUts, to escape finally from the atrial pore. 



In the early stages of development the gill-slits are ex- 

 posed on the sides of the body, and the water passes through 

 them directly to the exterior. Later on a longitudinal ridge- 

 like fold of the body-wall appears on each side of the body 

 above the gill-slits, and these atrial or epipleural folds rapidly 

 grow downwards so as to form flaps covering over the gUl- 

 sUts : the space between the atrial folds and the body- wall is 

 the atrial cavity. The flaps ultimately meet each other below 

 the body in the mid-ventral line, and fuse along their whole 

 length, except at the atrial pore, which remains open through- 

 out life. The anterior ends of the folds grow forwards to form 

 the side-walls of the buccal cavity. 



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. 41-43, 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 intestine and liver. 



On the primary gill-arches this layer of atrial epithelium 

 is wider than on the secondary arches, and towards the dorsal 



