T6 



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 



CD o HK 1 ^^ ^ 



Fig. 36.— The anterior end of anAmphioxus Larva with four primary gill-slits, 



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



CH, notoohord. DL, prseoral pit. BS, endostyle. GrD, aperture of duot of olub- 



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



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



P 6, sixth myotome 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. 



HK 14- 



HK 7 



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

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

 CH, notoohord. DL, prteoral pit. ES, endostyle. GL, club-shaped gland. 

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

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

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

 right metapleural fold, KC, neural canal. UO, anterior dilatation, or ventricle of 

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

 tliirteenth and fourteentli myotomes. 



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 



