362 



MANUAL OF ZOOLOGY 



SECT. XII 



is in some respects extremely primitive. The spinal col- 

 umn is represented merely by a thick persistent notochord, 

 enclosed in a sheath, with, in the lampreys, small carti- 



na..ap 



mlh 



oe.s.cl.d 



FIG. 222. Head of Myxine glutinosa (A) and of Bdellostoma forsteri (B), from 

 beneath, br. ap, branchial aperture; br. cl. i, first branchial cleft ; mth, mouth; 

 na. ap, nasal aperture; oes. ct. d, oesophageo-cutaneous duct. The smaller open- 

 ings in A are those of the mucous glands. (After W. K. Parker.) 



laginous processes representing neural and haemal arches. 

 The skull is cartilaginous, and is peculiarly modified. Be- 

 hind it in the lamprey is a remarkable basket-like apparatus, 



