472 



CYCLOSTOMATA, 



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 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 common exhalant ori- 

 fice. The gut is straight and 

 uniform, with wavy longitudinal 

 ridges internally, with a two-lobed 

 liver and a gall-bladder, but with- 

 out the usual pancreas. The 

 anus lies within an integumentary 

 cloacal chamber. 



Respiratory system. — Water 

 enters by the nasal sac, 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 re- 

 spiratory 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 cesophago- 

 cutaneous duct) opens from the 

 oesophagus to the exhalant aper- 

 ture, and represents a rudimentary 

 seventh pouch. 



Vascular system. — The blood 

 contains the usual amoeboid leuco- 

 cytes and red blood corpuscles, 

 elliptical in form (circular in the lamprey). It is collected 

 from the body in anterior and posterior cardinals, passes 

 through a sinus venosus into the auricle of the heart, thence 

 to the ventricle, thence along a ventral aorta, which gives off 

 vessels to the respiratory pouches. From these the purified 



Fig. 200. — Respiratory sys- 

 tem of hag, from ventral 

 surface. 



b., Barbules ; ?«., mouth opening 

 on ventral surface; g, gullet ; 

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

 to show internal lamellae ; g.p.*>, 

 sixth gill-pouch ; ex. , exhalant 

 canal of first gill-pouch ; v., 

 ventricle of heart ; ao., aorta ; 

 a., common exhalant aperture. 



