NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS I7 



antennae. The nauplius has only three pairs of limbs, and, since it 

 must swim, it has no choice but to use what appendages it has. As 

 the body lengthens and the postmandibular appendages become broad 

 and flat, these appendages assume the function for which they were 

 modified in the branchiopod ancestors. The antennae then revert to 

 a more simple form (fig. 3 E), and in the adult they are again modi- 

 fied, in the male (G) for grasping. 



OSTRACODA 



The ostracods, being enclosed in a bivalve shell from the time they 

 leave the egg, go through no body changes of form that might be 

 termed a metamorphosis ; their appendages, however, give an impres- 

 sive example of the extreme degree of structural modification that an 

 ordinary segmented leg may take on. 



The newly hatched ostracod larva is in the nauplius stage of de- 

 velopment (fig. 4 A), since it has only the three usual pairs of naupliar 

 appendages. It is not a typical nauplius, however ; the antennae and 

 mandibles are uniramous, and the body is already enclosed in a shell 

 formed in the egg. Here is a good demonstration, then, that the 

 crustacean nauplius, in addition to its primitive features, can take on 

 a specialized structure characteristic of the order to which it belongs. 

 During the postnaupliar stages, as shown in the series of drawings 

 (fig. 4) here copied from Schreiber (1922) on the development of 

 Cyprinotus incongrucns, the postmandibular appendages are succes- 

 sively added until the definitive number of seven in all is present in 

 the eighth instar (F), in which the larva has attained essentially the 

 adult structure. 



There is no question that the naupliar appendages are the anten- 

 nules, the antennae, and the mandibles, but there has been some 

 difference of opinion as to the identity of the postnaupliar appendages. 

 In the Cypridae the first appendage after the mandible (fig. 5 B, 4) 

 bears a large, flat, fringed lobe projecting upward in the shell cavity, 

 and this appendage is commonly regarded as the maxilla. The next 

 appendage (5) Schreiber termed the maxilliped. These two append- 

 ages on each side in Cypris arise side by side on the arm of the hypo- 

 stome (D, 4, 5), and Cannon (1926) regarded them as the maxillula 

 and the maxilla, respectively. In Limnocythere inopinata (A), how- 

 ever, as in other Cytheridac and in Nesideidae, appendage 5 is a 

 typical leg well separated from 4. If, therefore, appendage ./ is 

 interpreted as the maxilla, appendages 5, 6, and 7 are thoracic legs, 

 and Kesling (1951) says this is now the accepted interpretation of 



