l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



increased in length and are more distinctly indented on their mesal 

 margins. The slender posterior part of the body bears rudiments of 

 four new appendages, and its apex is split into a pair of small caudal 

 lobes. The postmaxillary appendages continue to develop through 

 the sixth and seventh instars until 1 1 pairs are present. In the eighth 

 instar (E) they have lost their leglike form and have become broad 

 flat phyllopodia with large flabella and slender apical lobes. At this 

 stage, as the thoracic appendages take over the swimming function, 

 the second antennae are much reduced in size and are directed for- 

 ward. Heath enumerates 17 instars in the larval life of Branchinecta, 

 but development beyond the eighth instar merely brings about refine- 

 ments toward the adult structure. 



The larval stages of Artemia described by Heath (1924) are very 

 similar to those of Branchinecta, as are those of Branchipus described 

 by Oehmichen (1921). In the Concostraca and Cladocera the larval 

 development is complicated by the formation of a bivalved shell. 



The development of the branchiopod appendages is of interest be- 

 cause it suggests that the natatory phyllopodium has been evolved 

 from a segmented ambulatory leg. The mature appendage of Branchi- 

 necta (fig. 3 F) is cut on its mesal margin into a number of lobes, of 

 which five (1-3) are commonly described as endites, while the large, 

 so-called flabellum (<5) is interpreted as the endopodite, and the mov- 

 able apical lobe as the exopodite. The same structure is seen in the 

 limbs of Branchipus (fig. 27 A, B) and other anostracans. Since 

 endites in general are lobes of the limb segments, the six mesal lobes 

 of the phyllopodium suggest that they represent six leg segments, 

 coxopodite to propodite. The movable, independently musculated 

 apical lobe (Dactpd), therefore, should be the dactylopodite. There 

 is thus in the phyllopodium evidence of the presence of the seven 

 segments characteristic of the crustacean walking legs. In the second 

 maxilliped of Apus (fig. 27 C) seven segments, including a terminal 

 dactylopodite, are plainly evident, and each of the first six segments 

 except the ischiopodite bears an endite. We can hardly escape the 

 conclusion, therefore, that the phyllopodial limbs of the branchiopods 

 have been evolved from 7-segmented walking legs. The metamorpho- 

 sis of the appendages, therefore, has taken place since the crustaceans 

 became crustaceans, and is recapitulated in the larval ontogeny. A 

 more extensive discussion of the nature of the primitive arthropod 

 limbs is given in section IV of this paper. 



About the only metamorphosis in the life history of Branchinecta 

 is the temporary adaptation of the antennae for swimming. It is 

 hardly to be supposed that the primitive crustaceans swam with their 



