3 22 



MANUAL OF ZOOLOGY 



SECT. 



which will be manifest shortly, is designated Acrania, the 

 rest of the sub-phylum being known as Craniata. 



A. THE AC RANI A 



This isolated group comprises only a single family, the two genera 

 (Branchiostoma, and Asymtnetron} of which are distinguished from one 



another by comparatively 

 slight differences. 



Branchiostoma 



(more widely known 

 under the name of Am- 

 pMoxUS)) the Lancelet, 

 , v, is a small transparent 



. 1;^" animal, occurring in the 

 sea near the shore and 

 burrowing in sand ; its 

 length does not exceed 

 5 '8 cm., or less than two 

 inches. Its form will be 

 obvious from Fig. 190. 

 The body is elongated, 

 pointed at either end, 

 and compressed. The 

 anterior two-thirds is 

 roughly triangular in 

 transverse section, pre- 

 senting right and left 

 sides, inclined towards 

 one another above, and 

 a convex ventral 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 longitudinal fold, the dorsal 

 fin (dors.f.)-. this is continued round the posterior end of the 



