76 



AMPHIOXUS. 



Fig. 37. The gill-slits are at first wide, window-like apertures 

 in the wall of the pharynx ; and, until the formation of the atrial 



UK 4 



HK I 



HK2 



HK 5 



FIG. 36. The anterior end of an Amphioxus Larva with four primary gill-slits, 

 from the left side. (After Lankester and Willey.) x 200. 



CH, iiotoehonl. DL, praeoral pit. ES, endostyle. GD, aperture of duct of club- 

 shaped gland. GL, club-shaped gland. HK 1, 2, 3, 4, first, second, third, and fourth 

 primary gill-slits. NS, spinal cord. O, margin of mouth opening. OC, eye-spot. 

 P 6, sixth niyotome of the left side. 



cavity, they open directly to the exterior. At a comparatively 

 early stage (Fig. 37), the first primary gill-slit becomes markedly 

 smaller than the succeeding ones. 



DL 



HK 14 



HK 7 



MD 



FIG. 37. The anterior end of an Amphioxus Larva with fourteen primary gill- 

 slits, seen from the right side. (After Willey.) 



CH, notochord. DL , praeoral pit. ES, endostyle. GL, club-shaped gland. 

 GO. opening from dub-shaped gland into pharynx. HK 1, 7, 14, first, seventh, and 

 fourteenth primary gill-slits. HP 2, 7, thickened patches in which the second and 

 seventh secondary gill-slits will be formed at a slightly later stage. MD, free edge of 

 right metapleural MJ. NC, neurnl canal. NO, anterior dilatation, or ventricle of 

 neural canal. M"S, spinal cord. O, mouth. OC, eye-spot. P 13, septum between 

 thirteenth and fourteenth myotornes. 



b. The secondary gill-slits, or the gill-slits of the adult right 

 side of the pharynx, appear later than the primary slits, and in 

 the following manner. At a stage (Fig. 37) when fourteen 



