ALIMENTARY AND RESPIRATORY SYSTEMS. 467 



union of dorsal and ventral roots is only partial, and there 

 is no sympathetic system. 



The eye is degenerate (e.g., without a lens or iris), and is 

 hidden beneath the skin ; the ear has only one semicircular 

 canal ; the single nasal sac opens dorsally at the apex of 

 the head, and communicates posteriorly with the pharynx 

 by a naso-palatine duct. The absence of pigment and 

 sensory structures in the skin, and the degeneracy of the eye 

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



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 "horny," but sharp. 

 Into the mouth, just in front of a fringed velum which 



FIG. 150. Median longitudinal section of anterior end 

 ofMyxine. (After RETZI us.) 



Showing barbules, nasal passage, mouth cavity, brain. 



separates it from the pharynx, the nasal sac opens. Thus 

 water passes from the nostril into the pharynx. It may 

 be, as Beard suggests, that this passage is a persistent u old 

 mouth." From the gullet open six respiratory pouches, 

 each of which has an efferent tube, but the six efferent 

 tubes of each side unite and have a common exhalent 

 orifice. The gut is straight and uniform, with longitudinal 

 ridges internally, with a two-lobed liver and a gall bladder, 

 but without the usual pancreas. 



Respiratory System. 



Water enters by the nasal sac, passes into the pharynx, 

 down the gullet, into the six pairs of respiratory pouches, 



