NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 57 



the primitive adult arthropod from which the Crustacea were evolved. 

 Life-history stages representing adult crustacean ancestors, therefore, 

 can be recapitulations only of forms that intervened in evolution 

 between the primitive arthropod and the modern crustacean. 



On the assumption adopted in the early part of this paper as a basic 

 concept, the primitive arthropod is presumed to have been an elongate, 

 segmented animal with a pair of similar jointed appendages on each 

 body segment (fig. i C). From such a progenitor all the modern 

 arthropods were evolved by special modifications, particularly of the 

 appendages, according to the adopted way of living. Ajmspides (D) 

 may be taken as an example of a fairly generalized modern crustacean, 

 but other crustaceans go through no developmental stage resembling 

 Anaspides or any other form that might be intermediate between 

 their adult structure and that of a primitive arthropod. The megalops 

 of a crab undoubtedly represents an early crab form, but there is 

 little evidence that the Crustacea in general recapitulate adult stages 

 of crustacean ancestry or the adults of other species of lower rank 

 in taxonomy. There is no reason to believe that the likeness of the 

 "mysis" stage of the penaeid (fig. 21 E, F) to an adult Mysis is any- 

 thing more than a superficial resemblance. Foxon (1936) has shown 

 that the decapod larvae do not go through a typical euphausiid or 

 mysid stage, and that neither the structure nor the function of the 

 mysid appendages is recapitulated in other groups. The precocious 

 development of the uropods before the pleopods are formed is ex- 

 plained by Foxon (1934) as an adaptation to reverse movement. 



Most crustaceans develop by anamorphosis, but the anamorphic 

 method of growth was established in the remote progenitors of the 

 arthropods before the arthropods became arthropods. The embryo 

 in the tgg goes through the preanamorphic stages of its ancestors, 

 and if it is hatched as a nauplius, the following ontogenetic stages 

 recapitulate the anamorphic steps of precrustacean evolution. The 

 larva, however, is destined to be a crustacean, it carries the genes of 

 its species, and its crustacean destiny is thus stamped on it before it 

 leaves the egg. Hence, from the beginning of its development the 

 larva takes on crustacean characters, but the forms it assumes are 

 ontogenetic and not recapitulations of adult crustacean evolution. 

 When the larva is set free at a very immature stage it must be struc- 

 turally adapted to the exigencies of an independent life, and it may be 

 modified for a way of living that was not at all that of its ancestors. 

 Thus the normal ontogenetic stages may take on metamorphic aberra- 

 tions having no relation to anything in the past history of the animal 



