LAR VAL DE VEL 0PM EN T. 



133 



SJC.C- 



r.m 



B 



Fig- 75- Transverse sections through the region of the mouth of larvce of 

 Amphioxus, to show the endostyle and the external and internal openings of club- 

 shaped gland. (After LANKRSTER and WILLEY.) 



A. Section passing through the anterior corner of the mouth of a larva, with 

 eleven gill-slits. 



B. Section passing through the middle of the mouth of a larva, with twelve 



gill-slits. 



al. Pharyngeal cavity, b.c. Ccelom (splanchnoccel) . br.e. Branchial epithelium. 

 e.a. Branchial artery, end. Endostyle. ex.o. External opening of club-shaped 

 gland. f.c. Dorsal fin-space, gl. Lower portion of club-shaped gland, g.s '. First 

 gill-slit. i.i. Interccelic membrane, in.o. Internal opening of club-shaped gland. 

 La.. Left aorta; there is no corresponding right aorta in the larva, tii. Mouth. 

 r.m. Rudiment of right metapleur; a mere ectodermic thickening in A ; a solid 

 thickening of the cutis in B, in which two of the original enlarged ectoderm cells 

 have become imbedded. These curious cells occur over a long stretch of the 

 metapleural folds during this phase of the development, disappearing eventually. 



In B, the left metapleur is indicated by an ectodermic thickening immediately 

 below the gill-slit, x. So-called nephridium of Hatschek, 



