FROM EGG TO ADULT 299 



adult insect is passed in a realm quite different from that 

 of the larva. The mature dragon flies, May flies, and 

 hellgramite are inhabitants of the air, while the larval 

 young dwell in the water. The adult cicadas love the 

 pure air and sunshine, but their larvae are subterranean 

 wanderers. 



Many of the crustaceans exhibit a complete metamor- 

 phosis, and in numerous species three or four stages are 

 discernible. In the case of the small crustaceans (Ento- 

 mostraca), including the barnacles and shrimps, the larval 

 form hatching from the egg is known as the nauplius. It 

 differs from the adult in having usually but one eye and 

 three pairs of leglike appendages which later become trans- 

 formed into the antennules, antennae, and mandibles. 

 The body is not segmented. In the barnacles a second 

 larval stage occurs, called the cypris, characterized by a 

 two-valved shell, six pairs of swimming feet, and paired 

 compound eyes. Certain of the larger crustaceans (Mala- 

 costraca) exhibit two stages in development known as 

 zoea and my sis. The larval stages last but a few days, 

 and in any species are passed through in the egg. 



Some of the mollusks, especially the bivalves, pass 

 through a larval state known as the veliger. It is a minute, 

 free-swimming form. In the case of the fresh-water mussel 

 a second larval stage occurs, while as a parasite it is at- 

 tached to a fish. The eggs of the sea-urchin and starfish 

 do not hatch directly into forms resembling the parents, 

 but give rise to young differing as widely from the adult 

 as the caterpillar differs from the butterfly. 



Even among vertebrates, as in the case of some fish and 

 all amphibians, the development from egg to adult is 



