762 LIFE : OUTLINES OF GENERAL BIOLOGY 



half an inch long and leave the stone nest. They burrow into the 

 sand or mud and remain larval for three or four years, differing 

 markedly from the adults, e.g. in having the eyes hidden, in having 

 no teeth, and in the horseshoe shape of the fringed mouth-opening. 

 When 4-6 inches long and 3-5 years old, they undergo a remarkable 

 metamorphosis, putting on the adult characters. The young forms 

 of the river lampreys remain in fresh water, while those of the marine 

 species {Peiromyzon marinus) go down to the sea, or it may be to a 

 great lake. After two or three years the marine forms are sexually 

 mature and may be a yard long. They return to the rivers to breed, 

 and the reproductive processes of producing and liberating eggs 



Fig. 123. 



Megalopa Larva of a Crab, with the abdomen in a line with the cephalothorax. 



After Roule. 



and sperms are so exhausting that both sexes die after the spawning 

 season. In this case there is a long-drawn-out larval period, and 

 though the adult life is not short, it is abruptly terminated by a 

 coincidence of reproduction and death. The crest of the life-curve is 

 followed by an abrupt vertical descent. 



In the life-history of the European salmon there is a lengthening 

 out of several chapters. The eggs are liberated late in the year on the 

 gravelly bed of the river and covered among the pebbles; they are 

 more or less fortuitously fertilised by the attendant male; there is 

 a long-drawn-out embryonic development during the cold months, 

 for cell-division is sluggish when the temperature is low; the eggs 

 are hatched after three months or so — in marked contrast to the 

 lamprey's. Out of the eggs come "alevins", still weighed down by 



