XII 



PHYLUM CHORDATA 



323 



bers of the sub-phylum, and are best regarded as consti- 

 tuting by themselves a division, which, for reasons which 

 will be manifest shortly, is designated Acrania, the rest of 

 the sub-phylum being known as Craniata. 



A. THE ACRANIA 



This isolated group, the Acrania, comprises only a single family, the 

 two genera (Br&nchiostoma and Asymmetrori) of which are distin- 

 guished from one another by comparatively slight differences. 



Branchiostoma (more widely known under the name of Amphioxus) , 

 the lancelet, is a small transparent animal, occurring in the sea near 

 the shore and burrowing in sand; its length does not exceed 5.8 cm. 



FIG. 203. Amphioxus lanceolatUS. A. ventral; B, side view of the entire animal. 

 an, anus; atrp, atriopore; cd. f, caudal fin; ctr, cirri; dors, f, dorsal fin; 

 dors.f. r. dorsal fin-rays; gon, gonads; mtfll, metapleure; ntyotn, myomeres; 

 nek, notochord; or. kd, oral hood; vent, f, ventral fin; vent. f. r, ventral fin- 

 rays. (After Kirkaldy.) 



or less than two inches. Its form will be obvious from Fig. 203. The 

 body is elongated, pointed at either end, and compressed. The anterior 

 two-thirds is roughly triangular in transverse section, presenting right 

 and left sides, inclined towards one another, above, and a convex ven- 

 tral surface. The posterior third is nearly oval in section, the right 

 and left sides meeting above and below in a somewhat sharp edge. 

 Extending along the whole of the dorsal border is a median longi- 

 tudinal fold, the dorsal Jin {dors, /"); this is continued round the 



