CHARACTERS OF FISHES 589 



three-chambered, and receives pure blood from the lungs as 

 zvell as impure blood from the body. Apart from the Dipnoi, 

 the heart has a single auricle receiving impure blood from the 

 body, and a ventricle which drives this through a ventral 

 aorta to the gills, whence the purified blood flows to the head 

 and by a dorsal aorta to the body. In addition to the 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 Dipnoi, there is no vein which resembles 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 the kidneys, and others from the posterior region. Its 

 place is taken by paired posterior cardinals. 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. 



Compared with Cyclostomes, the true fishes show a distinct 

 advance. Thus we may note — the jaws formed from the 

 first visceral arch, the limbs, the dermal exoskeleton of scales, 

 the frequent occurrence of bone, the true teeth, the paired 

 nostrils, the three semicircular canals, the renal-porta 

 system, the spleen, and the genital ducts. 



First type of Fishes. The Skate (Raja) — one of the 



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 

 Hve 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- 



