28 ANATOMY OF AMPHIOXUS. 



the pharynx is perforated by the gill-clefts, it is necessary 

 for it to have some sort of skeletal support to prevent it 

 from collapsing. This is effected by a series of stiff gelat- 

 inous rods which lie in the walls bounding; the gill-clefts. 

 These rods have the consistency of chitin, — the material 

 that forms the exoskeleton of insects, — and are insoluble 

 in caustic potash. The portion of the pharyngeal wall 

 which lies between any two gill-slits is called a gill-bar. 



It will be seen at once in Fig. lo that there are two 

 kinds of skeletal rods differing in the behaviour of their 

 lower extremities. Dorsally the rods arch over into one 

 another, but ventrally they are independent, and every 

 alternate rod is bifurcated, while the somewhat shorter 

 intermediate rods end plainly. The forked rods form the 

 skeletal support of the priinary gill-bars, while the inter- 

 mediate simple rods support the secondary gill-bars, or 

 to)igiie-bars, as they are usually called. The primary bars 

 constitute the walls of the primary gill-clefts. The latter, 

 at their first origin, appear as simple oval openings in 

 the wall of the pharynx. Later on the simple opening 

 becomes divided into two by the gradual dipping down- 

 wards of its dorsal margin until it meets and fuses with 

 the ventral margin. In this way is the tongue-bar formed 

 and the gill-slit doubled. (Cf. Fig. ii.) The statement 

 which was made above, therefore, that there could be as 

 many as i8o openings on each side of the pharynx, signified 

 that there might be some ninety pairs of primarv gill-clefts. 



Eventually the gill-slits become still further subdivided, 

 though not so obviously, by the formation of small cross- 

 bars which pass over from one primar\- bar to another, 

 skipping over the tongue-bar, although eventuallv fusino- 

 with the skeletal axis of the latter on their inner faces 

 (Fig. 10). 



