VERTEBRATA ACRANIA. 141 



The gill-slits slope downwards and backwards, so that a number 

 of them are cut through by a single transverse section. They 

 are separated by lath-shaped gill-arches, the flat surfaces of which 

 face one another and are covered by very long cilia. New gill- 

 slits are continually being added, in growing animals, at the 

 posterior end of the series as the pharynx increases in length. 

 Each gill-slit is at first a simple oval aperture, but soon becomes 

 horseshoe-shaped and finally divided into anterior and posterior 

 parts by the downgrowth of a tongue-like secondary gill-arch 

 from its dorsal margin. The 1st, 3rd, 5th, &c., arches are conse- 

 quently secondary, and the alternate ones primary. It is also to 

 be noted that the gill-slits are bridged over by numerous short 

 horizontal bars, so that the lateral walls of the pharynx resemble 

 open basketwork in structure. 



There is a somewhat complex branchial skeleton supporting the 

 parts described, and composed of firm material, conveniently 

 termed chitinoid, since it resembles horn or chitin in physical 

 respects, though its exact chemical nature is not known. Each 

 gill-arch is traversed by an internally grooved rod of this kind, 

 which in the primary arches is solid and forked at both ends, 

 but in the secondary arches hollow and simple ended. These 

 rods are connected by short horizontal pieces which run through 

 the bars that bridge the gill-slits, and ventrally they come into 

 relation with a double series of small chitinoid plates supporting 

 the floor of the pharynx. 



The pharynx is succeeded by an exceedingly short gullet, and 

 this again by a fairly wide stomach, from which a simple blindly 

 ending tube, the liver, runs forwards along the right side of the 

 pharynx, extending further in adult than in young specimens, 

 such as the one drawn in Fig. 38. The stomach passes gradually 

 behind into a tubular intestine running straight to the anus. 



Amphioxus feeds chiefly upon small organisms suspended in 

 the surrounding water, and in the normal position of the animal 

 i.e., vertically imbedded in the sand, with its anterior end 

 projecting, these are swept into the mouth by the currents 

 which the ciliated lining of the gut sets up. A certain amount of 

 sand also appears to be swallowed for the sake of the contained 

 organic matter. Since the anterior part of the Chordate gut has 

 to do with respiration as well as nutrition, it is commonly special- 

 ized so that the food takes a different path from the respiratory 



