■10 AMPHIOXUS. 



jaws. The buccal orifice opens into a buccal cavity (Fig. 11, a), 

 which is bounded laterally by the buccal hood, and posteriorly 

 by a muscular diaphragm, the velum ; a small perforation in 

 the velum, a little way below its middle, is the true mouth and 

 leads into the pharynx. 



The pharynx (Fig. 11, c) is a wide sac, forming about half 

 the length of the alimentary canal, and attached along its mid- 

 dorsal line to the under surface of the sheath of the notochord, 

 (Fig. 12). The sides of the pharynx are perforated by a large 

 number of slit-like apertures, the gill- slits, which run obliquely 

 downwards and backwards, and of which in the adult animal 

 there may be one hundred or more on each side. The parts of 

 the pharyngeal wall left between 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 second, fourth, &c, 

 or primary arches, being forked at their ventral ends, while 

 the rods of the alternate, or secondary arches, are unsplit. Each 

 double gill-slit is originally a single one, but becomes divided 

 in the course of development (vide p. 78), by the downgrowth 

 of the unsplit bar, or tongue-bar as it is termed, from its dorsal 

 end. 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. 



Along the mid-dorsal line of the pharynx is a deep epibran- 

 chial groove (Fig. 12, f), lined by a single layer of long columnar 

 ciliated cells. A band of similar cells, the endostyle (Fig. 12, g), 

 runs along the mid-ventral wall of the pharynx; it is folded 

 longitudinally in its hinder part to form a groove (Fig. 13, g). 



The intestine (Fig. 11) commences at the hinder end of the 

 pharynx, close to the dorsal surface ; and runs straight back to 

 the anus, which is on the ventral surface, some little distance 

 from the hinder end of the body, and slightly to the left of the 

 median plane. The intestine is extremely narrow at its com- 

 mencement ; further back it dilates to form an expanded part or 

 stomach, from which a large pouch-like outgrowth, the liver 

 (Fig. 11, d), extends forwards some distance along the right side 

 of the pharynx, ending blindly in front. 



During life a stream of water passes through the mouth 

 into the pharynx, and then out through the gill-slits in the sides 



