NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 9 



up to the adult. The embryo and the early larva, therefore, represent 

 pre-crustacean stages of arthropod evolution. The embryo, however, 

 must reproduce its parental form. Hence the crustacean characters 

 appear at a very early stage of ontogeny, but the resulting embryonic 

 or larval stages are not recapitulations of adult crustacean evolution. 

 The crustacean characters are simply precociously imposed on the 

 anamorphic stages of ontogeny. Finally, if the embryo is set free as a 

 larva at an early stage of development, it must be structurally adapted 

 to a free life, and in its subsequent growth other adaptations may 

 be necessary. Thus it comes about that metamorphosis still further 

 complicates the course of ontogeny. The life histories of parasitic 

 larvae best demonstrate that larval forms are metamorphic adaptations 

 to a way of living, since the nonparasitic adult ancestors of such spe- 

 cies can hardly be supposed to have had the larval form. Where a 

 specialized adult structure has arisen since the crustaceans became 

 crustaceans, there may be a true recapitulation of an earlier adult 

 form, as in the megalops of the crabs. A further discussion of the 

 nature of larval forms will be given in connection with the life history 

 of a penaeid (p. 54). 



II. THE NAUPLIUS AND THE METANAUPLIUS 



Since among the crustaceans the young hatch at different periods of 

 development, the youngest larvae may have very diverse forms in the 

 various orders, representing different ontogenetic stages according to 

 the degree of development they undergo within the egg. The earliest 

 hatched larval form is the nauplins, which is particularly characteristic 

 of the Entomostraca, but occurs also in the Euphausiacea and Pe- 

 naeidea among the Malacostraca. The nauplius is usually followed by 

 a metanaiiplius, which is the first stage of postembryonic growth. 

 From the metanauplius on, development may be merely a matter of 

 regular anamorphic growth by the successive addition of new segments 

 and appendages, but in many species the larva takes on different forms 

 as it develops. These ontogenetic changes differ so much in the various 

 orders that no general description can be given, hence a discussion of 

 them will be left to the next section of this paper. Special attention, 

 however, must be given to the nauplius and the metanauplius. 



The nauplius. — The nauplius is a minute creature, highly variable 

 in form in different species, but typically ovoid or pyriform in shape 

 with the larger end anterior (fig. 2 A). It has a pair of uniramous an- 

 tennules, or first antenna (lAnt), typically biramous second antennae 

 (2 Ant) and mandibles (Md), and a median eye of two or more parts. 



