MYXINE 



499 



may be, as in the lamprey, a combination of olfactory and 

 pituitary involutions. The absence of pigment and sensory 

 structures in the skin, and the simple state of the eye and 

 ear, may be partly associated with the hag's mode of life. 

 It seems probable that the simplicity 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 

 the pharynx. It may be, as Beard 

 suggests, that this passage is a per- 

 sistent "old mouth," the palseo- 

 stoma 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 common exhalant ori- 



fice. The gut is straight and 



FIG. 239. Respiratory sys- 

 tem of hag, from ventral 

 surface. 



liver and a gall-bladder, but with- *> Barbuies ; ;., mouth opening 



,. on ventral surface; g., gullet; 



The 



uniform, with wavy longitudinal 

 ridges internally, with a two-lobed 



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

 to show internal lamellae ; g.p.^, 

 sixth gill-pouch; e.t., exhalant 

 canal of first gill-pouch; v., 

 ventricle of heart ; ao., aorta; 

 a., common exhalant aperture. 



out the usual pancreas 



anus lies within an integumentary 



cloacal chamber. 



Eespiratory system. - - Water 

 may enter by the nasal sac or 

 by the mouth. It passes into the pharynx, down the gullet, 

 into the six pairs of respiratory pouches and their efferent 

 tubes, and leaves the body by the single aperture at each 

 side. The respiratory pouches have much-plaited internal 

 walls, on which the blood vessels are spread out. On the 

 left side, behind the sixth pouch, a tube (the uesophago- 



