422 



THE LANCELET 



CHAP. 



only the atriopore 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 larvce 



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

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



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. f. r, vent. f. r). 



In the course of development, each primary gill-slit 

 becomes divided into two by the growth, from above 



