36o 



MANUAL OF ZOOLOGY 



Bdellostoma there are six pairs of small gill-slits, in Myxine 

 only a single aperture on each side (Fig. 208, br. ap.\ 



The skeleton is very unlike that of the true Fishes, and 

 is in some respects extremely primitive. The spinal column 

 is represented merely by a thick persistent notochord, en- 

 closed in a sheath, with, in the Lampreys, small cartila- 

 ginous processes representing neural and haemal arches. 



br.cl.i 



na.cz.jo 



FIG. 207. Petromyzon marinus. Ventral (A), lateral (B), and dorsal (C) views 

 of the head. br. cl. /, first gill-cleft , buc. f. buccal funnel ; eye, eye ; mth. 

 mouth ; no., ap. nasal aperture ; /. papillae ; /. pineal area ; fl. ft. fi. teeth 

 of buccal funnel ; t*. teeth of tongue. (After W. K. Parker.) 



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

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

 composed of cartilaginous bars. This branchial basket, as it 

 is termed, supports the gill-sacs. 



The gill-sacs, of which there are either six or seven pairs, 

 are the organs of respiration, representing the gills of the 



