422 



THE LANCELET 



sides, pushing the ccelome 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 resembling 



S. Awphioxus lanceolatus. Diagrammatic transverse sections of larvae 



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

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

 dorsal fin-ray ; tn. 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. coelome. (From Korschelt and 

 Heider's Embryology, after Lankester and Willey.) 



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. />, 

 vent. f.r). 



In the course of development, each primary gill-slit 

 becomes divided into two by the growth, from above down- 



