THE LIFE OF FISHES. 415 



much smaller number, including many sharks, live and usually 

 breed in the open sea ; about 100 genera have been recorded 

 from the great depths. In regard to the latter, Giinther has 

 shown that in fishes firom a depth of 80-200 fathoms the eyes 

 are larger than usual, as if to make most of the scanty light ; 

 beyond the 200 fathom line small-eyed forms occur with 

 highly developed organs of touch, and large-eyed forms which 

 have no such organs, but seem to follow the gleams of 

 phosphorescence ; finally, in the greatest depths blind fishes 

 occur with rudimentary eyes. Many of these abyssal fishes 

 are phosphorescent ; the colouring is usually simple, most 

 being blackish or silvery ; the skin exudes much mucus ; 

 the skeleton tends to be light and brittle ; the forms are 

 often very quaint ; the diet is necessarily carnivorous. 



The Skate {Raja) — a type of Fishes, especially of the order 

 Elasmobranchii. 



Various species of Raja — the skate proper {R. batis), the 

 thornback {R. clavatd), and the ray {R. maculata) — are 

 common off the coast of Britain. They differ only in colour 

 and minute details. They are comparatively sluggish but 

 undoubtedly 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. No one who has looked at a skate will require any 

 description of the triangular snout, the broad pectoral fins, 

 the long tail with small unpaired fins ; but the following 

 external characters should be verified. On the dorsal surface 

 the skin is pigmented and studded with skin-teeth ; on the 

 top of the skull there are two unroofed areas or fontanelles, 

 and there are numerous jointed rays 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 sensitive mucus-canals 

 extending over the skin, the transverse mouth and the 



