418 COLLEGE ZOOLOGY 



sturgeon, cod, and mackerel. With its rasp-like tongue a 

 hole is made in the flesh of the victim and the blood sucked 

 out. 



The mouth opens into a buccal cavity (Fig. 355, m). Two 

 tubes lead posteriorly from the latter, a dorsal oesophagus (ces) 

 and a ventral respiratory tube (r.t), guarded by a fold called the 

 velum (vl). There is no distinct stomach. The oesophagus is 

 separated from the intestine (int) only by a valve. A fold in the 

 intestine called the typhlosole (see also p. 219) forms a sort of 

 spiral valve. A liver (Ir) is present, but there is usually no bile 

 duct (b.d.) in the adult. 



The Circulatory System. — - Petromyzon possesses a heart, 

 a number of veins and arteries, and many lymphatic sinuses 

 (Fig. 355, s, s'). The heart lies in the pericardium (pc), and 

 consists of a ventricle (v) which forces the blood into the arteries 

 and an auricle (au) which receives the blood from the veins. A 

 renal portal system is absent. 



The Respiratory System. — Respiration is carried on by means 

 of seven pairs of gill pouches (Fig. 355, br.j), which open to the 

 outside by the gill-slits (e.a) and internally to the respiratory 

 tube (r.t). Each gill pouch contains numerous gill lamellce, 

 (br.fj). Water is taken into the gill-sacs through the gill-slits, 

 and discharged by the same openings. 



The Nervous System. — The brain (Fig. 355, br.) of the adult 

 lamprey is very primitive and in many respects similar to that 

 of the embryos of higher vertebrates. It is remarkable because 

 of its thin membranous roof and the small band-like cerebellum. 

 The spinal cord is flat, and lies on the floor of the neural canal 

 (n.ca.). 



The Sensory Organs. — Organs of taste, smell, hearing, and 

 sight are present in the lamprey. The end organs of taste are 

 situated between the gill pouches on the pharyngeal wall. The 

 organ of smell is an olfactory sac (Fig. 355, na) which lies in the 

 nasal capsule (Fig. 354, 5) and opens by a nasal aperture (Fig. 

 355, na") on the dorsal surface between the eyes. The olfactory 



