26 PHYLUM CEPHALOCHORDA. 



diverticulum, the liver, which pushes before itself the ventral 

 body wall, the whole projection lying in the atrial cavity along- 

 side the pharynx on its right side (Fig. 10). This process is 

 attached to the dorsal wall of the atrium in front. The lining 

 cells of the hepatic caecum are coloured green in the living 

 animal, as are the cells of the stomach from which it is given off. 

 Outside the intestinal epithelium is a thin connective tissue 

 layer which may contain unstriated muscular fibres. 



The atrial cavity is a space lined by ectoderm and surround- 

 ing the pharynx and anterior part of the intestine ventrally and 

 laterally. It opens to the exterior at the hind end of the ventral 

 groove at the level of the 36th myotome. It extends back on 

 the right side behind the atrial pore almost as far as the anus. 

 Its lining cells are in part ciliated and pigmented with a brown 

 pigment. The dorsal wall of the atrium is folded, in consequence 

 of the fact that it is reflected on to each of the primary 

 bars at a point considerably ventral to that at which 

 it joins the secondary bars (Fig. 12, Id). This gives rise 

 to somewhat puzzling features in transverse sections in 

 which the dorsal regions of the primary bars appear to 

 be connected to the side walls of the atrium by strands 

 of tissue traversing the atrium (Fig. 10). The ligarnentum 

 denticulatum of J. Miiller is this folded roof of the atrium 

 cut longitudinally. It follows from this arrangement that 

 the atrium is prolonged further dorsally in the region of the 

 tongue bars than it is over the primary bars. 



The brown canals are two tubes lined by a pigmented epithe- 

 lium, and projecting into the dorso-pharyngeal coelom (Fig. 10). 

 They lie parallel to the long axis of the body and probably 

 end blindly in front at the 27th myotome. Posteriorly at the 

 level of the junction of the pharynx and intestine they open by 

 funnel-shaped apertures into the dorsal part of the atrial cavity, 

 one on each side. They are to be regarded as forwardly directed 

 diverticula of the atrium, of unknown function. 



These structures are often called the atrio-coelomic funnels. They 

 were discovered by Lankester. It is doubtful whether they end blindly in 

 front or open into the dorso-pharyngeal coelom. 



There is a well- developed coelom with which the gonads are 

 in relation. A complete comprehension of the coelom cannot 



