DIGESTIVE SYSTEM. 193 



3. The pharynx is a wide sac, forming about half the length 

 of the alimentary canal. It is attached along its 

 mid-dorsal line to the under surface of the sheath of 

 the notochord, and is almost surrounded by a space 

 the atrial cavity which receives the water that 

 passes out of the pharynx. Its sides are perforated by 

 a large number of slit-like apertures the gill-slits 

 which run obliquely downwards and backwards. The 

 parts of the pharyngeal wall between the successive 

 slits are narrow bars the gill-arches each of which 

 is strengthened by an axial rod of a chitinous sub- 

 stance. These arches are of two kinds, arranged 

 alternately : the axial rods of the third, fifth, etc., 

 arches being forked at their ventral ends, while the 

 alternate rods are unsplit. Each double gill-slit is 

 originally a single one, but becomes divided in the 

 course of development by the downgrowth of the 

 unsplit bar from its dorsal end. The split bars may 

 hence be called primary rods, and the unsplit ones 

 secondary rods. Like the myotomes, the gill -arches 

 are not in pairs, but alternate on the two sides 

 of the body. 



The successive gill-arches are connected by 

 horizontal bars, of which there are usually three or 

 more crossing each slit, so that the pharynx has the 

 character of an open meshwork. The inner surface 

 of each gill-slit is lined by columnar flagellate 

 cells, which maintain a continual stream of water 

 through the gill-slits from the pharynx to the atrial 

 cavity, the stream serving to aerate the blood in 

 the vessels of the arches as it passes over them. 

 The inner edge of each bar also bears flagellate 

 cells separated from those lining the gill-slits by 

 two groups of cells bearing very short cilia. 



Along the mid-dorsal line of the pharynx is the 

 deep epibranchial groove, lined by a single layer 

 of long columnar ciliated cells. A band of similar 

 cells runs along the endostyle or mid-ventral wall 



