3*2 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- 



oes.cl.d 



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



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

 na. ap, nasal aperture ; oes. ct. d, cesophageo-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, 



