120 CHILDHOOD OF ANIMALS 



In most of the marine worms very many eggs are laid, and 

 these develop into free-swimming larvse, subject to the usual 

 destruction by innumerable foes. In a few the number is re- 

 duced and the young are carried attached to the body of the 

 mother, sometimes contained in a pair of brood-pouches on the 

 under surface of her body. In terrestrial and fresh- water worms, 

 such as the common earthworm, the number of young is reduced, 

 and these are enclosed in little protective cases or cocoons formed 

 by the parent and suspended to water- weeds, or hung up against 



the wall of the burrow. 

 Leeches arrange for 

 their young in a similar 

 fashion, but there are 

 some in which the eggs 

 remain attached to the 

 ' body of the mother, 

 to the under surface of 

 which the young leeches 

 fix themselves when 

 they are hatched, by 

 their suckers, and so 

 secure protection. 



All the crustaceans 

 have gone a consider- 



FiG 27 A Brittle-star carrying its young. g-ble way in the reduC- 



(Ajter Wyville Thomson.) . , , 



tion of the number of 



eggs produced and all of them display at least the beginnings of 



parental care. In a very few, including some of the fish-lice, 



the eggs are attached by the mother to water-weeds or stones. 



In most they are carried about by the female in a brood-pouch, 



or attached to the legs or to a special chamber formed from 



the flap that protects the external gills. When the eggs hatch, in 



most cases free-swimming larvae emerge, and these without further 



aid from the parents are transformed to the adults by a series of 



moults. During their larval life, however, prodigious numbers are 



destroyed, for crustacean larvae form a most important part of the 



food-supply of fishes and aquatic birds, and the different groups 



supply many cases of a still greater protection of the young by the 



parents, with reduction in the number produced and a much higher 



percentage of success in reaching adult life. The summer eggs of 



the little water-fleas (Cladocera) are hatched in a brood-pouch 



