ALIMENTARY SYSTEM 305 



pharynx passes into the mid-gut, from which a large diverticulum 

 runs forwards on the right side of the pharynx. This diverti- 

 culum is probably the chief digestive gland, since its cells are 

 glandular and food does not enter it. All three classes of food- 

 stuff are digested together in the mid-gut in an approximately 

 neutral medium. The cilia of the posterior end of the mid-gut 

 rotate the cord of mucus, and there is a short hind-gut which 

 runs back to the anus. Absorption of food takes place chiefly in 



Dorsal 



Vmtral 



Fig. — 231. Three nephridia of Branchio stoma lanceolatuni in situ, drawn with 

 camera lucida from a stained preparation. — From Goodrich, 1933, Quart. 

 Journ. Micr. Sci. 75. 



c, suprapharyngeal coelom ; ct., cut edge of atrial wall; d., diverticulum of nephridial canal ; n., neph- 

 ridium; p., nephridiopore ; PB., primary gill bar; s., solenocyte ; SB., secondary gill bar. 



the hind-gut, but there is perhaps also some intracellular 

 digestion in the mid-gut. 



nephrid;[A 



The lancelet possesses a series of nephridia, similar in their 

 main structure to those of the more primitive annelids, lying 

 between the wall of the atrium and the dorsal coelomic canals. 

 They correspond in number and position to the primary gill 

 slits. Each is a bluntly branched tube (Figs. 231, and 232), with 

 an opening below into the atrium, and dorsally a number of 

 elongated fiame-cells or solenocytes, which project into the coelom. 

 The solenocyte is a single cell, with a nucleus in the knobbed end, 



