LARVAL FISHES : THEIR METAMORPHOSES. 123 



neck, yet it can move fast in water by means of 

 its sculling tail." 



In more than one instance these larvae have 

 been mistaken for adult species, their immature 

 condition being unsuspected. 



A case in point illustrating this is afforded 

 by one of the lowest of the fishes — the fresh- 

 water lamprey {Petromyzon planeri). For a long 

 while the young of this was regarded as a dis- 

 tinct species, the ammocetes. Its true nature 

 was discovered by a German ichthyologist, Aug. 

 Miiller. The young ammocetes, like the typical 

 tadpole larva of our text, has a sucker-like mouth 

 devoid of teeth, and in many other respects 

 differs from the adult form. It is further re- 

 markable in that the full-grown larva may even 

 be larger than the adult ! Its larval life is a 

 very prolonged one, lasting often as long as five 

 years. Its transformation into the adult form 

 seems to be as sudden as it is radical. Amongst 

 the more important of these changes are the 

 introduction of conical horny teeth, and the de- 

 velopment of the eyes, which in the larval form 

 lie beneath the skin, like those of the young of 

 many of the higher vertebrates, e.g. : the cat, dog 

 and rabbit. Changes in the form of the skeleton, 

 of the gill-pouches, and of the alimentary canal 

 and kidneys also take place, and are changes of 

 great significance. We might mention here that 

 the adult life of the lamprey is very brief, and 

 terminates directly after the deposition of the 

 eggs or sperms, as the case may be. 



The sucker-like mouth, or suctorial mouth, as 

 we may more conveniently call it, is a feature of 



