MABINE VEETEBEATES 309 



There are three or four British lampreys, two or three of which 

 inhabit fresh water. Their habits do not seem to be well under- 

 stood, but it appears certain that the Sea Lamprey (Petromyzum 

 marinus), which reaches a length of from one to two feet, ascends 

 rivers to spawn, while the smaller River Lamprey (P.fluviaiilis) 

 has been caught in the sea ; and it is probable that the migrations 

 of both, together with the sojourn of the young of the former for a 

 longer or shorter period in fresh waters, have been the cause of the 

 widespread confusion between species. 



Lampreys are carnivorous creatures, and attach themselves to 

 fishes by their suctorial mouths, and rasp away the flesh. They 

 have also been known to attack bathers. 



Passing now to the true fishes, we must first study the general 

 features of the group by which they are to bo distinguished from 

 other animals. Since there are so many creatures outside this class 

 that are more or less fishlike in some respects, it becomes no easy 

 matter to give a concise definition of a fish, and the shortest satis- 



Fio. 226. THE SEA LAMPBEY 



factory description must necessarily include several points of struc- 

 ture. Thus, we may define a fish as a cold-blooded vertebrate that 

 does not undergo metamorphoses, with limbs modified into fins, 

 possessing also median fins on the dorsal and ventral surfaces, 

 having distinct jaws, a heart with two chambers, and breathing by 

 gills. To this we may add that the young are generally produced 

 from eggs, and that the skin is covered with scales or bony plates, or 

 is naked. 



But let us now look more closely into the structure of fishes, so 

 that we may be enabled to see how marvellously they are adapted 

 to their aquatic life, and in order that we may become acquainted 

 with the few technical terms which will, as a matter of convenience, 

 be used in the descriptions of species. 



Taking first the external features, we note that the body is 

 generally covered with scales, sometimes very large and distinct, 

 but often so small and closely set that they are not visible without 

 careful examination ; indeed they are often so small, and so 



