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 (Branchiostoma and Asymmetron) of which are distin- 

 guished from one another by comparatively slight differences. 



Branctaostoma (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. 



alrp ventfr 



B myom dorsj'r 



dorsf 



77iyom 



-or.hd 



jon 



jnlpl 





<=df 



Fig 201. -Amphioxus lanceolatUS. A. ventral; B, side view of the entire animal. 

 «« anus; atri, atriopore; cd. /, caudal fin; cir, cim; dors, f, dorsal fin. 

 dors./, r. dorsal fin-rays; gon, gonads; mtil, metapleure; myom, myomeres, 

 nch, notochord; or. hd. oral hood; vent. /, ventral fin; vent /. 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 fin {dors. /); this is continued round the 



