Adjustment of the Life Cycle 



261 



so also is the pupal shelter of the caterpillar of Elophila 

 fulicalis; hardly less so is the portable case of the larva 

 of the caddis-fly, Leptocerus ancylus or of Molanna 

 angustata. 



ADJUSTMENT OF THE LIFE CYCLE 



Life runs on serenely in the depths of the seas where, 

 as we have noted in Chapter II, there is no change of 

 season; but in shoal and impermanent waters it meets 

 with great vicissitudes. Winter's freezing and summer's 

 drouth, exhaustion of food and exclusion of light and of 

 air, impose hard conditions here. Yet in these shoals 

 is found perhaps the world's greatest density of popula- 



Fig. 162. The flattened and limpet-shaped cases of Ithytrichia 

 confusa, as they appear attached to the surface of a sub- 

 merged stone. 



