DIGESTIVE SYSTEM. 185 



projecting cirri. This aperture appears to corre- 

 spond to the mouth of higher vertebrates. 



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, from which it hangs down freely into 

 the atrial cavity. 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 substance. 

 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 hori- 

 zontal 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 waler 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 



