FISHES 



423 



Round-Mouths (Cyclostomata). — The only member of this 

 group of which the life-histories and habits have been carefully 

 studied are the Lampreys (species of Petromyson), of which some 

 are marine, others live in fresh water, while others again spend 

 part of their adult existence in the sea and part in rivers. And 

 even the marine forms enter fresh water for the purpose of 

 spawning. We have three native species, the large Sea- Lamprey 

 {Petromyzon marimis), the much smaller River -Lamprey or 

 Lampern {P. fluviatilis), and the diminutive Small Lamprey {P. 

 Planeri). In all of them the eggs are deposited in a sort of 

 trench which is dug out for 

 their reception, and hatch 

 out in the form of a larva 

 which differs so much from 

 the adult that it was for- 

 merly believed to be a dis- 

 tinct species, and received 

 the name of Ammocoetes 

 (fig. 949), by which it is 

 still known. The larva is 

 blind and toothless, and is 

 said to swallow mud for the 

 sake of the contained organic 

 matter. After several years 

 of larval existence, during which it undergoes a gradual series 

 of changes, it assumes the adult form, and the development of 

 horny " teeth " enables it to adopt a carnivorous habit. The 

 eyes, which are at first useless and covered by the skin, come 

 to the surface, and other changes in the internal structure also 

 take place. 



In the case of the Sea- Lamprey and the Lampern, when larval 

 life is over there is a migration to the sea, and there is no return 

 to fresh water except for spawning purposes. It is said that the 

 Lampern usually dies after laying its eggs. Regarding the habits 

 of the Small Lamprey there is considerable divergence of opinion. 

 Some observers believe that its entire life is spent in its native 

 brook or river, while others assert that it migrates to the sea like 

 the other two species. There can be little doubt that it dies after 

 spawning, being thus reminiscent of what takes place in many 

 Insects, where the greater part of life is spent in the larval state. 



lowci- lip 



Fig, 949. — Front end of Ammocoetes from Left Side (a) and 

 from below (b), enlarged. ?£., nostril; g.a. (b), gill apertures. 



