. Outlines of zoology. Zoology. 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 unifor
. Outlines of zoology. Zoology. 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 ; ', first gill-pouch, cut open to show internal lamellae ; *>, sixth gill-pouch ; ex., exhalant canal of first gill-pouch ; v., ventricle of heart ; ao., aorta ; a., common ex
Size: 1025px × 2438px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
