272 THE STUDY OF ANIMAL LIFE CHAP, xm 



canal at the back of the mouth, and serves for the inflow- 

 ing of water ; the six gill-pockets on each side open 

 directly into the gullet, but each has an excurrerit tube, 

 and the six tubes of each side open at a common aperture. 

 The animal lives away from the light, and its eyes are 

 rudimentary, hidden beneath skin and muscles. The 

 skin of the irritated animal exudes so much slime that the 

 ancients spoke of the hag " turning water into glue." 



In several ways the hag is strange. Thus J. T. Cun- 

 ningham discovered that it is hermaphrodite, first pro- 

 ducing male elements, and afterwards eggs, and Nansen 

 has corroborated this. The eggs are large and oval, 

 each enclosed in a " horny ' ' shell with knotted threads 

 at each end, by which a number are entangled together. 

 How they develop is unknown. The hags devour the bait 

 and even the fish from the fisherman's lines, and three 

 or four are sometimes found inside a hooked fish. 



5. Fishes. Fishes are in the water as birds in the air 

 swift, buoyant, and graceful. They are the first back- 

 boned animals with jaws, while scales, paired fins, and 

 gills are their most characteristic structures. The scales 

 may be hard or soft, scattered or closely fitting, and are 

 often very beautiful in form and colour. The paired 

 fins are limbs, as yet without digits, varying much in 

 size and position, and helping the fish to direct its course. 

 The gills are feathery outgrowths of the wall of the 

 pharynx bordering the gill-clefts, and supported by gill- 

 arches. They afford a large surface on which the blood- 

 vessels are washed by the water. They are the breathing 

 organs of all fishes, but in the double-breathing mud- 

 fishes (Dipnoi) the swim-bladder has come to serve as 

 a lung, and there are hints of this in a few others. 



There are several subdivisions of fishes : 



(1) The cartilaginous fishes (Elasmobranchs or Sela- 

 chians) are for the most part quite gristly, except in 

 teeth and scales. Among them are the flattened skates 

 and rays with enormous fore-fins, while the sharks and 

 dogfish are shaped like most other fishes. Their pedigree 

 goes back as far as the Silurian rocks, in which remains 

 of shark-like forms are found. A Japanese shark (Chla- 



