LAH VAL DE V 'EL 0PM EN T. 



133 



Fig. 75- — Transverse sections through the region of the mouth of larva? 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.m. Intercoelic membrane, in.o. Internal opening of club-shaped gland. 

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

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

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

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

 metapjeural folds during this phase of the development, disappearing eventuallv. 



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

 below the gill-slit. x. So-called ncphridium of Hatschek. 



