CHAP, xvi Backboned Animals 253 



The sea lamprey (P. marinus] may measure three feet ; the 

 river lamprey (P. fluviatilis} about two feet ; the small lampern or 

 stone -grig (P. branchialis or planeri] about a foot. The flesh is 

 well known to be palatable. 



The glutinous hag (Myxine glutinosa) is an eel -like animal, 

 about a foot in length, of a livid flesh colour. It is common at 

 considerable depths (40 to 300 fathoms) off the coasts of Britain 

 and Norway, and, when not feeding, lies buried in the mud with 

 only its nostril protruded. Like the lamprey, it has a smooth 

 slimy skin, a gristly skeleton, a round suctorial mouth with teeth. 

 The single nostril communicates with the food-canal at the back 

 of the mouth, and serves for the inflowing of water ; the six gill- 

 pockets on each side open directly into the gullet, but each has an 

 excurrent 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 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. Cunningham 

 discovered that it is hermaphrodite, first producing 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 some say that they bore 

 their way into living fishes such as cod. 



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

 buoyant, and graceful. They are the first backboned animals with 

 jaws, while scales, paired fins, and gills are their most character- 

 istic 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 outgrowths of skin with a plaited surface, on which the 

 branching 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 at least four orders of fishes : 



(i) The cartilaginous fishes (Elasmobranchs or Selachians) 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- 



