STRUCTURE OF MYXINE 



577 



until they enter the brain ; the ear has only one semi- 

 circular canal. The single nasal sac (with paired folds of 

 olfactory epithelium in Bdellostoma, an American relative) 

 opens dorsally at the apex of the head, and communicates 

 posteriorly with the pharynx by a naso-palatine duct. It 

 may be, as in the lamprey, a com- 

 bination of olfactory and pituitary 

 involutions. The absence of pig- 

 ment and sensory structures in the 

 skin, and the simple state of the 

 eye and ear, may be partly associ- 

 ated with the hag's mode of life. 

 It seems probable that the sim- 

 plicity is primitive rather than 

 degenerate. 



Alimentary system. — The 

 mouth is suctorial. There is a 

 median tooth above, and two 

 rows of teeth are borne on each 

 side of the muscular " tongue." 

 These teeth are entirely " horny," 

 but sharp. Into the mouth, just 

 in front of a fringed velum which 

 separates it from the pharynx, the 

 nasal, or, as some would say, the 

 naso-pituitary, sac opens. Thus 

 water passes from the nostril into -pio. 325.— Respiratory sys- 

 the pharynx. It may be, as Beard tem of hag, from ventral 

 suggests, that this passage is a 

 persistent *' old mouth," the 

 palaeostoma of Kupffer. From 

 the gullet open six respiratory 

 pouches, each of which has an 

 efferent tube, and the six efferent 

 tubes of each side unite in a com- 

 mon exhalant orifice. The gut is straight and uniform, 

 with wavy longitudinal ridges internally, with a two-lobed 

 liver and a gall-bladder, but without the usual pancreas. 

 The anus lies within an integumentary cloacal chamber. 



Respiratory system. — Water may enter by the nasal 

 sac or by the mouth. It passes into the pharynx, down the 



37 



surface. 



b., Barbules ; m., mouth opening 

 on ventral surface ; g., gullet ; 

 g.p'., first gill-pouch, cut open 

 to show internal lamellae ; g-p.^, 

 sixth gni-pouch ; ex., exhalant 

 calial of first gill-pouch ; v., 

 ventricle of heart ; ao., aorta ; 

 a., common exhalant aperture. 



