THE SKATE. 477 



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 exactly corresponds to what is known 

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

 single vessel which receives hepatic veins from the liver, renal 

 veins from the kidneys, and other veins from the posterior 

 organs. 



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. 



Various species of Raja the grey skate (R. batis], the 

 thornback (R. clavatd), and the ray (R. maculata) are 

 common off the coast of Britain. They are comparatively 

 sluggish but very voracious fishes, and live along the bottom 

 at considerable depths. 



Form. 



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 pigmented and studded with 

 placoid scales or skin teeth ; on the top of the skull there 

 are two unroofed areas or fontanelles ; there are numerous 

 jointed radials in the pectoral fins. Behind the lidless eyes 

 are the spiracles the first of the obvious gill slits, opening 

 dorsally, containing a rudimentary gill, and communicating 

 posteriorly with the mouth cavity. On the ventral surface, 

 are seen the sensory mucous canals extending over the skin, 

 the transverse mouth and the nostrils incompletely separated 

 therefrom as if in double hare lip, the five pairs of gill 

 apertures, the cloacal aperture and two abdominal pores 



