14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



by the successive addition of segments and appendages, many of their 

 changes are merely those resulting from the anamorphic manner of 

 growth. In nearly all cases, however, there is some degree of meta- 

 morphosis superposed on the anamorphic stages, varying from a 

 mere adaptation of the appendages for swimming to a total recon- 

 struction of the animal for a parasitic way of life. The most striking 

 examples of crustacean metamorphosis, therefore, occur in parasitic 

 species. Among the Crustacea metamorphosis evidently has been de- 

 veloped separately in each order, and often independently in different 

 members of the same order. There is no type of metamorphosis 

 characteristic of large groups of orders, as in the holometabolous 

 orders of insects. Moreover, since crustacean metamorphosis affects 

 the juvenile anamorphic stages, except where it is carried over into 

 the adult, the metamorphosis of Crustacea has no relation to that of 

 the epimorphic insects. A brief but interesting account of the life- 

 history problems of crustacean larvae is given by Gurney (1926). 



For much assistance in preparing this section of the paper the writer 

 is indebted to Dr. Fenner A. Chace, Jr., and his associates in the di- 

 vision of marine invertebrates of the U. S. National Museum. 



BRANCHIOPODA 



The branchiopods undergo few changes during their larval de- 

 velopment that are not the result of simple anamorphic growth by 

 which the body and the appendages are completed and brought to the 

 adult condition through successive instars. The branchiopods are 

 thus of interest in showing a simple progressive development from 

 nauplius to adult, which is accompanied, however, by a specialization 

 of the postgnathal appendages for swimming. As an example we may 

 take the life history of Branchinecta occidentalis Dodds as described 

 by Heath (1924). 



The newly hatched larva of Branchinecta is a typical nauplius 

 (fig, 3 A) with three pairs of appendages, a median simple eye, and 

 a large labrum, but the oval, unsegmented posterior part of the body 

 is more than usually constricted from the forepart. The large second 

 antennae are the principal swimming organs. Between the nauplius 

 and the second instar, or metanauplius (B), a very considerable 

 change takes place. Lateral compound eyes are now conspicuous by 

 their pigmentation. The posterior part of the body has greatly length- 

 ened, and bears rudiments of maxillulae, maxillae, and six or seven 

 following pairs of appendages. In the third instar (C) the post- 

 maxillary appendages have lengthened and the more anterior pairs 



