4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



In the second case segmentation appears later, usually progressing 

 from before backward, suggesting that it represents a former ana- 

 morphic mode of segment formation in which the anterior segments 

 are the oldest. Since anamorphic growth, either in the larva or the 

 embryo, is characteristic of the annelid worms and recurs in so many 

 of the arthropods, it was probably the primitive method of growth in 

 the annulate animals. 



The most immature larval form among the arthropods is the crus- 

 tacean nauplius. For practicable purposes early hatching must be 

 given up by terrestrial animals, unless they go back to the water to 

 lay their eggs, as do the land crabs, frogs, and toads. The anamorphic 

 myriapods do not quit the egg until they have acquired the adult type 

 of structure and are equipped with a sufficient number of legs for 

 terrestrial locomotion. The completely epimorphic spiders and insects 

 are best fitted to cope at once on hatching with the conditions of their 

 environment, and they have become the most successful of the land 

 arthropods. Though some insects lay their eggs in the water and the 

 young are aquatic, they are simply terrestrial forms that have become 

 secondarily adapted in the larval stage for life in the water; they 

 hatch at the same stage as their relatives on land. 



The Crustacea are primarily aquatic animals ; only a few have be- 

 come adapted to a permanent life on land. The eggs of most species 

 are laid in the water, and the newly hatched young must be capable 

 of swimming; the adults can later adopt a bottom habitat if they ac- 

 quire ambulatory legs. Considering the uniformity of the water 

 environment of a swimming larva, there is relatively little inducement 

 for a young aquatic animal to undergo adaptive metamorphoses. The 

 metamorphoses of most crustacean larvae, therefore, are relatively 

 simple as compared with those of insect larvae, which have a great 

 diversity of habitats open to them. Parasitic crustaceans, however, 

 are a conspicuous exception to this generalization. 



As a rule small animals in the water are eaten by larger animals, 

 but the small creatures have one recourse against their possible preda- 

 tors and that is to become parasitic on them. Parasites, however, 

 have to be structurally adapted to a parasitic life, and consequendy 

 most parasites undergo metamorphic changes. jMany of the smaller 

 crustaceans have adopted parasitism, and the most extreme degrees 

 of crustacean metamorphosis are found among such species, especially 

 if the adults themselves remain parasites. Such adults in some cases 

 have lost all resemblance to the ancestral forms of their race, even 

 every mark of their crustacean ancestry. Moralists may cite the 

 "degeneration" of such parasites as a warning of what parasitism may 



