326 THE STUDY OF ANIMAL LIFE CHAP. 



is undoubtedly a great achievement ; the relatively long 

 caterpillar period makes the ecstasy of the butterfly 

 possible. 



In many other life-histories we hear, so to speak, the 

 same tune. There is a lengthening out of the larval 

 period, a vantage-ground is slowly attained from which 

 the adult form can begin afresh on different, usually more 

 evolved, lines. The May-flies or Ephemerides previously 

 referred to (p. 112) are often almost diagrammatic, for 

 many of them have two or three years of sub-aquatic 

 larval life and two or three days (or less) of aerial and 

 reproductive activity. 



In the sea-lamprey (Petromyzon marinus) there is a 

 somewhat similar punctuation of life, but with a notable 

 improvement, that the adult life is longer. The eggs are 

 laid in a cleared space in the bed of a river and adhere to 

 the sand and small pebbles ; in about a fortnight ciliated 

 larvae emerge which burrow in the sand or mud and feed 

 on small aquatic animals ; they differ from the parents 

 in the horseshoe shape of the mouth, in being blind, in 

 the details of their respiratory system, and in some other 

 ways. They remain larva? for three or four years, after 

 which they undergo a metamorphosis, losing their larval 

 features and putting on those of the adult. They leave 

 the fresh water and spend two or three years of vigorous 

 life and rapid growth in the sea, feeding exclusively on 

 fishes. Eventually they return to the rivers to spawn- 

 the male shedding the seminal fluid or milt upon the 

 eggs just as they are laid by the female. It is noteworthy 

 that the setting in of reproduction is associated with a 

 stoppage of nutrition, and the life-story, differing from 

 that of the May-flies in the duration of the adult stage, 

 ends in the same way, for death rapidly follows the 

 spawning. The curve ends in the same way an almost 

 vertical drop after reproduction. 



In the case of the common eel there is also a greatly 

 elongated larval period, lasting it may be for a couple 

 of years. Near the surface of the open sea where the 

 European continental mass slopes down to the great 

 abysses, and also in the mid-Atlantic, transparent, knife- 



