522 



CEPHALOCHORDA 



the adult, forming the system of lymph spaces and canals which lie 

 below the cutis. In the ventral portions the septa disappear, and the 

 enclosed spaces, bounded by somatic mesoderm and splanchnic meso- 

 derm, imite to form the " splanchnocoel " which surroimds the gut. 



Fig. 298. — Sections through embryos of Amphioxus, to illustrate 

 development of the body cavity. — After Hatschek. 



On the upper line, three longitudinal sections ; on the lower line, 

 three transverse sections, ec, Ectoderm ; en., endoderm ; a., 

 archenteron ; p.s., primitive segments (protovertebrae) ; n.c, 

 nerve-cord ; p., posterior end ; np., neuropore ; ne.c, neuren- 

 teric canal ; m.p., medullary or neural plate ; ch., notochord ; 

 ep., splanchnocoel — above it is the myocoel. 



In the adult this space is reduced anteriorly to small spaces and 

 coelomic canals, by the development of the atrial chamber (see Figs. 297 

 and 298). The coelomic spaces and canals contain coagulable fluid, 

 and represent the lymphatic system of higher forms. 



Besides the main trunk portion of the coelom, there is an anterior 

 portion, which is separated off from the very front of the gut, and is then 



