THE CEPHALOCORDA 181 



sheath and lying in a cavity which at first sight looks like the 

 peritoneal space or coelom. This cavity, however, is not 

 coelom, but atrium, and is lined by an epiblastic epithelium 

 easily distinguished from coelomic epithelium because of the 

 presence of brown pigment granules in its cells. Examining 

 the section closely, we find that the hypoblastic wall of the 

 intestine is invested by three layers of tissue. Externally there 

 is the atrial epithelium, and within this, and closely adherent to 

 it, a thin sheet of mesoblastic tissue which we may call the 

 somatopleur. Then follows a narrow space; then another 

 very thin layer of mesoblastic tissue, the splanclmopleur, 

 which closely invests the hypoblastic epithelium of the gut. 

 The space between the somatopleur and splanchnopleur is the 

 coelom. Below, and at the sides of the intestine, it is so narrow 

 as to be scarcely recognisable, but dorsally the splanchnopleur 

 and somatopleur diverge slightly from one another, so that the 

 ccelomic space is more obvious. On either side of the middle 

 line the splanchnopleur turns upwards and outwards, and is 

 continued into the somatopleur, so that the gut appears to be 

 suspended from the notochord by a double membrane, which 

 is, in fact, a very short mesentery. The two layers of the 

 mesentery are not in contact, but are separated by a lymph 

 space, in which lies the dorsal aorta. 



Similar relations are seen in a section taken between the 

 atriopore and the anus (fig. 43, B\ but here the atrium is 

 reduced to a diverticulum lying on the right side only of the 

 intestine, so that the description given above applies to that 

 side only. On the left side the somatopleur is adherent to 

 the connective tissue covering of the myotomes. 



Throughout the pharyngeal region the intestine and its 

 coverings are perforated by the gill-slits, and as a consequence 

 the coelom, instead of being a continuous though narrow space 

 surrounding the gut on all but the dorsal side, is subdivided 

 into dorsal and ventral sections connected by canals running 

 in the primary gill-bars. The manner in which these sub- 

 divisions and connections were formed requires a little thought. 

 Before the gill-slits or atrial chamber were formed the pharynx 

 lay, like the intestine, in a crelomic space, and at first in a 

 relatively large space, for it was not compressed by the 

 presence of the atrial chamber subsequently formed round it. 

 The gill-slits were formed as a succession of oval apertures, 



