470 



SUB- PHYLUM CEPHALOCHORDA. 



right side, and they are strongly asymmetrical. The mouth 

 is placed at the left side ; the gill-slits of one side appear 

 considerably before those of the other; the primitive seg- 

 ments of one side are not opposite those of the other, and 



FIG. 254. Sections through embryos of Amphioxus^ to 

 illustrate development of body cavity. 



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

 three transverse sections, ec., Ectoderm; en., endbderm ; a. t 

 archenteron ; /.J., primitive segments (protovertebrae) ; n.c. t 

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

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

 cp., splanchnocoel above it is the myocoel. 



so on. By the process known as the " symmetrisation " of 

 the larva, the apparent symmetry of the adult is produced. 

 The adult position of the anus and of the olfactory pit, 

 both to the left side, and the position of the unpaired liver 

 diverticulum, show how partial this process is. 



