78 



AMPHIOXUS. 



HK.I2 



wall, finally assuming their permanent position on the left side 

 of the pharynx. During the process of shifting, the primary 

 and secondary slits gradually become equal in size, and of similar 

 shape. From the dorsal border of each slit a small process, the 

 tongue-bar, grows downwards across the slit, dividing it into 

 anterior and posterior portions ; these tongue-bars (Fig. 38) 

 appear rather earlier in the secondary than in the primary slits. 

 Of the fourteen primary slits, the first and the fourteenth 

 close up and disappear; and at slightly later stages the thir- 

 teenth, twelfth, eleventh, and tenth similarly, and in succession, 

 close and disappear (cf. Fig. 39). In this way the primary gill- 



OB 



HT HK 2 HKJ 



FIG. 39. The anterior end of an Amphioxus Larva with twelve primary gill- 

 slits, of which the first and twelfth are disappearing, and eight secondary 

 gill-slits ; seen from the ventral surface. (After Willey.) 



CH, notochord. ES, endostyle. HK1, first primary gill-slit just before its final 

 disappearance. HK 2, second primary gill-slit. HK 12, twelfth primary gill-slit, in 

 the act of closing, prior to its disappearance. HP 1, 8, first and eighth secondary gill- 

 slits. HT, tongue-bar. LM, velum. OB, buccal cavity. OT, buccal tentacles. 



slits become reduced to the same number, eight, as the secon- 

 dary slits, the eight persisting primary slits being the second to 

 the ninth inclusive. 



The anterior persisting slits of both series, i.e. the second 

 primary slit and the first secondary slit, differ from the others 

 in their smaller size, and in the fact that they alone do not 

 develop tongue-bars (Fig. 39, HK 2 ; HP i). 



The gill- slits have now reached the condition characteristic 

 of the critical stage. Eight slits are present on each side of the 

 pharynx, alternating with one another as in the adult ; the 

 anterior slit of the right side, i.e. the first secondary slit, HP i, 

 being opposite the interval between the first and second slits of 

 the left side, i.e. the second and third primary slits. 



