280 VERTEBRATA : PISCES OR FISHES. 



tions into seven cavities or pouches which receive water 

 from a canal which is distinct from the oesophagus, and 

 which has as many lateral holes as there are cavities ; and 

 they have seven branchial or gill openings on each side of 

 the neck ; and the nostril is single, tubular, and on the top 

 of the head. But one of the most curious features of these 

 animate is the mouth ; this is circular, and the tongue 

 moves forward and backward in it like a piston, enabling 

 them in this way to produce a vacuum, and thus to fix 

 themselves firmly to stones qr other bodies. The word 

 Petromyzon, the name of the genus which contains most 

 of these animals, comes from two Greek words, petros, a 

 stone, and muzo, to suck. They often attach themselves 

 to various substances. 



Some species of the Lampreys live in the sea, and 

 ascend rivers to lay eggs ; others live wholly in fresh- 

 waters. 



The Hags or Myxinidse, like the Lampreys, have a cir- 



FIG. 351. 



Hag or Myxine, Myxine limosa, Girard. 



cular mouth, and a tongue which acts as a piston in ex- 

 hausting the air from the mouth, thus enabling them to 

 fix themselves firmly to other fishes. 



The Ampliioxus or Lancelot is the lowest of all the 

 vertebrates. So little does it appear like a vertebrate, 

 that Pallas, the naturalist who first described it, supposed 

 it to be a slug or sort of snail-like animal. It is found 



O 



among the rocks of the sea-coast, and is about one or two 

 inches in length. It is partially transparent, has no 



