THE SKATE. 531 
blood, except in the Dipnot, where it shows hints of becoming 
three-chambered, and receives pure blood from the lungs as 
well as impure blood from the body. Apart from the Dipnot, 
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 
Sulbus arteriosus at the base of the ventral aorta. Except 
in Dipnot, 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 
Srom the kidneys, and others from the posterior region. Its 
place is taken by paired posterior cardinals. The kidney ts 
usually a persistent mesonephros. 
There ts no distinct indication of an outgrowth from the 
find 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-portal 
system, the spleen, and the genital ducts. 
First type of Fisues. The Skate (#aj2)—one of the 
Elasmobranchii 
The smooth skate (2. datis), the thornback (2. clavata), 
and the ray (2. 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 
