NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 3 



form directly into the adult, but most of them first undergo a recon- 

 struction in a special, proimaginal pupal stage. Among the Crustacea 

 tliere is no transformation stage strictly comparable to the insect pupa. 



True metamorphic forms are not recapitulations of phylogenetic 

 stages in the evolution of a species. An insect larva, though often 

 wormlike in appearance, does not represent a worm stage in the an- 

 cestry of insects. A caterpillar has a modern insect head and mouth 

 parts, a well-developed tracheal system, and wings growing beneath 

 its cuticle. No worm, ancestral or otherwise, was ever thus equipped. 

 Among the Crustacea also most juvenile forms assumed during the 

 larval growth are temporary adaptations to a changed mode of life 

 and are not phylogenetic recapitulations. Yet, it is true that former 

 ancestral characters discarded somewhere along the line of evolution 

 may appear in the ontogeny of the individual, and it is often difficult 

 to determine what phases of development are recapitulatory and what 

 are metamorphic aberrations. The following hypothetical example 

 wuU make clear the distinction between the two, and will lead to a 

 practical definition of metamorphosis. 



If the eggs of birds regularly hatched into reptilelike creatures, 

 which later transformed into feathered fowls, the change of form 

 would literally be a metamorphosis ; but, since birds have been derived 

 from reptilian ancestors, it might be specifically a case of phylogenetic 

 recapitulation. On the other hand, if there issued from the bird's egg 

 a creature having no relation to anything in the avian line of adult 

 evolution, but which still finally transformed into a bird, the change 

 of form would be one of quite a different nature, and it is this kind 

 of change that will be regarded as metamorphic in the following dis- 

 cussions. As here defined, therefore, metamorphosis is a structural 

 change at any time in the life history of an animal that may be re- 

 garded as an aberration from the ancestral direct line of adult develop- 

 ment which followed approximately the phylogenetic course of evolu- 

 tion of the species. In this case metamorphosis may affect the embryo, 

 the larva, or the adult. Simple development without metamorphic 

 interpolations might then be termed orthomorphosis. 



In the higher Crustacea there is a tendency for hatching to take 

 place at later and later stages of ontogeny, leaving a correspondingly 

 lesser amount of development to be accomplished after the larva leaves 

 the tgg. Finally a condition is reached when body segmentation and ap- 

 pendage formation are complete or almost so at hatching ; the animal 

 then becomes epimorphic in its development. In an epimorphic arthro- 

 pod, the embryonic development may proceed by the method of ana- 

 morphosis, or the entire body may be first laid down as a germ band. 



