482 PISCES— FISHES. 



two essential chambers of the heart, there is a sinus venosus, 

 which serves as a porch to the auricle, and there is often a 

 muscular conus arteriosus in front of the ventricle, or a bulbus 

 arteriosus at the base of the ventral aorta. Except in £)ipnoi, 

 there is no vein which exactly corresponds to what is known in 

 all higher Vertebrates as the inferior vena cava, i.e. a single 

 vessel receiving hepatic veins from the liver, renal veins from 

 kidneys, and others from the posterior region. The kidney is 

 usually a persistent mesonephros. 



There is no distinct indication of an outgrowth from the 

 hind end of the gut comparable to that which forms the 

 bladder of Amphibians or the allantois of higher Vertebrates. 



Most fishes lay eggs which are fertilised in the water. 



First type of Fishes. The Skate {Raja) — of the order 

 Elasmobranchii. 



The smooth skate (R. batis), the thornback (R. clavata), 

 and the ray (R. maculata), and other species are common 

 off British coasts. They are very voracious fishes, and live 

 on the bottom at considerable depths. 



External characters. — The body is flattened from above 

 downwards or dorso-ventrally, unlike that of the bony flat- 

 fishes, such as plaice and flounder, which are flattened 

 from side to side. The skate rests on its ventral surface, 

 the flounder on its side. The triangular snout, the broad 

 pectoral fins, the long tail with small unpaired fins, are 

 obvious features. On the dorsal surface the skin is pig- 

 mented and studded with placoid scales ; on the top of the 

 skull there are two unroofed areas or fontanelles ; numerous 

 jointed radials support the pectoral fins. Behind the lidless 

 eyes are the spiracles — the first of the obvious gill-slits, 

 opening dorsally, containing a rudimentary gill, and com- 

 municating posteriorly with the mouth cavity. On the 

 ventral surface are seen the sensory mucus canals, the 

 transverse mouth and the nostrils incompletely separated 

 from it, as if in double harelip, the five pairs of gill 

 apertures, the cloacal aperture and two abdominal pores 

 beside it. Pectoral and pelvic girdles support the fore- and 

 hind-fins. In the male the hind-fins are in part modified 

 into complex copulatory "claspers." 



