THE VERTEBRATA 151 



a stone by means of its suctional mouth, thus 

 fixing itself at rest. The Hag-fish, Afyxine, in 

 many respects similar, devours dead fishes chiefly. 

 The Hag-fish is found on English coasts: so is 

 the Marine Lamprey ; while two freshwater forms 

 are found in streams. 



Leaving the Cyclostomata, as the above fishes 

 are called, we reach the true fishes, which have 

 limbs and scales. Something has already been 

 said regarding their teeth and gills. The Car- 

 tilaginous fishes, in which most part of the skele- 

 ton remains gristle and does not become trans- 

 formed into bone, include the Sharks, Rays, and 

 Dog-fishes, all savage animals with strong teeth. 

 The common spotted Dog-fish of our own shores 

 is familiar to everybody : fishermen regard it with 

 disgust, as it is not eatable. The Rays are flat- 

 tened fishes, which live at the bottom of rather 

 deep water, and attain enormous size even on our 

 own coasts. The Thornback Skate is covered 

 with prickles (placoid scales). All these fishes 

 are grouped under the name of Elasmobranchii, 

 the Strap-gilled, so called from the structure of 

 the gill-arches. 



The majority of familiar fishes, such as the 

 herring, mackerel, cod and sole, belong to the 

 group of Teleostei, or Bony Fishes, in which, by 

 contradistinction from the last group, as much of 

 the skeleton as possible becomes bone. Never- 

 theless, traces of the notochord persist in the 

 back-bone of these fishes. Break the back-bone 

 across, of a cod or a sole, and you will find be- 

 tween adjacent sides of the centra, or middle 

 parts of the vertebrae, a pad of gristly substance. 

 This is the remaining substance of the notochord, 

 which finds room between the cup-shaped sides of 



