5 1 o PfSCES- -FISHES. 



t 



the heart has a single auricle receiving impure Hood 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 

 conns 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 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. 



First type of FISHES. The Skate (Raja) of the order 



Elasmobranchii. 



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

 and the ray (R. maculatd], 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 



