n6 



PISCES 



number of uniform teeth succeeding each other throughout life, a 

 separate opening for each gill -slit, an optic chiasma, a contractile 

 conns arteriosus, a purely venous heart, a pericardium communicat- 



Fio. 77. 



A. stomach and intestine of Ckirocentrus dorab, Forsk., opened up to show the spiral valve. 

 B, three portions of the intestine of Alopias vutyes, M. and H., cut longitudinally, c, caecum ; 

 e. w, cut wall ; e, entrance of pyloric end ; i, intestine; /■, pyloric end of stomach ; si, stomach; 



s.v, spiral valve. The upper piece is cut through on the near side of the middle line ; the 

 lower pieces through the middle line. 



ing with the abdominal coelom, a spiral intestinal valve, a 'free' 

 ovary, a well-developed oviduct, and a cloaca. 



The Pisces are such an ancient class, their remains having been 

 found even in the Lower Silurian rocks, and so many side twigs of 



