ATEIAL CAVITY 175 



D. The Atrial Cavity, 



The boundaries and relations of this cavity can only be 

 determined by the examination of a series of tr'ansverse 

 sections. 



The atrial or epipleural cavity is the large space which 

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

 respiratory stream of water passes from the pharynx through 

 the gill-slits, 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 gill- 

 slits : 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. 32-34, 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 

 surface of the pharynx it rises up into folds. Two or more of 

 these folds unite on each side to form the suspensory folds of 

 the pharynx, which run across from the dorsal part of the 

 sides of the pharynx to the inner surface of the atrial folds, 

 and separate the atrial cavity below from the dorsal co3lomic 

 canals above. 



The atrial cavity commences opposite the anterior end of 

 the pharynx, and extends back almost as far as the anus : 



