422 



THE LANCELET 



CHAP. 



only the atrioporc open, it gradually surrounds the 

 pharynx at the sides, pushing the coelome before it, so 

 that the latter becomes greatly reduced in this region 

 (Fig. 109, A, co). 



The skeleton is very simple ; besides the notochord 

 there are rod-like bars of a substance somewhat 





FIG. 108. Amphioxus lanceolatus. Diagrammatic transverse sections of larva 



in three different stages to show the development of the atrium. 

 ao. aorta ; c. derm ; ch. notochord ; d. intestine ; /. fascia ; fh. cavity for dorsal 

 fin-ray ; m. myomere ; n. spinal cord ; p. atrium ; sf. metapleural folds ; 

 sfh. metapleural cavity; si. sub-intestinal vein ; sk. wall of neural canal and 

 sheath of notochord; si. sub-atrial ridge; sp. coalome. (From Korschelt and 

 Heider's Embryology, after Lankester and Willey.) 



resembling cartilage supporting the oral hood and cirri 

 (Fig. 107, sk), chitinoid rods (br. r) supporting the gill- 

 bars between the clefts, and short rods of connective- 

 tissue, the fin-rays, in the dorsal and ventral fins (Figs. 

 106 and 107, dors. /. r, vent. f. r). 



In the course of development, each primary gill-slit 

 becomes divided into two by the growth, from above 



