396 



CEPHALOCHORDA 



[CH. 



coelic region but they become invaded by the lower ends of the trunk 

 myotornes ; the ventral muscle however which runs across the under 

 surface of the atrial cavity (Fig. 188), and which by contracting 

 diminishes the size of this cavity and thus expels water, originates 

 from the walls of the splanchnocoel. 



The mouth is originally some distance behind the anterior end, 

 and on the left side, so that there is a prae-oral portion of the body 

 which in the embryo is occupied by an anterior division of the 



ci 



g.s eiid 



FIG. 191. Anterior portion of body of young 

 transparent Amphioxus. After J. Miiller, slight- 

 ly altered. 



ch. Notochord. ci. Oral cirri. e. Eye-spot. 

 end. Endostyle. /.r. Fin-rays. g.s. Gill- 

 slits ; the skeletal rods of the gill-bars are 

 indicated by black lines, nt. Spinal cord, with 

 pigment granules near its base. r.a. Down- 

 growth from right aorta lying to the right 

 of vel. the velum, with velar tentacles pro- 

 jecting back into pharynx. w.o. Ciliated 

 epithelial tracts on inner surface of oral 

 hood. 



coelom corresponding to the proboscis 

 cavity of the Hemichorda. Subse- 

 quently however the atrial flaps extend 

 right to the anterior end, so that a new 

 terminal mouth is formed leading into a 

 chamber which is clothed by ectoderm 

 and which is therefore to be regarded as 

 the stomodaeum. The. opening of the 

 stomodaeum now forms the apparent 

 mouth, and the lips of this secondary 

 mouth grow out into rods supported by 

 gelatinous material and covered with cilia, 

 the so-called oral cirri, the function 



^ 



K 



FIG. 192. Anterior portion of 

 spinal cord ofAmphioxus ; 

 seen from above. After 

 Schneider. 



Between the first pair of 

 cranial nerves is seen the 

 eye-spot; one of the 

 branches of the second 

 pair of cranial nerves 

 sometimes arises directly 

 from the spinal cord as 

 shown on the right ; far- 

 ther back are seen the pig- 

 ment spots of the nerve- 

 cord. 



