466 SCHIZOPODA. 



veloped, though without appendages, and the tail, which functions 

 in swimming, is usually forked. The six posterior thoracic seg- 

 ments are, on the other hand, rudimentary or non-existent. There 

 are seven anterior pairs of appendages shewn in detail in fig. 211, 

 viz. the two pairs of antennae (At. I. and At. //.), neither of them 

 used as swimming organs, the mandibles without a palp (ma 7 ), 

 well-developed maxillae (two pairs, mx \ and mx 2), and two or 

 sometimes (Macrura) three pairs of biramous natatory maxilli- 

 peds (mxp I and mxp 2). Two lateral compound stalked eyes 

 are present, together with a median Nauplius eye. The heart 

 has in the majority of cases only one or two (Brachyura) pairs of 

 ostia. 



The Zoaea larva, though typically developed in the Decapoda, 

 is not always present (e.g. Astacus and Homarus), and some- 



mxp.2\ '' / At J 



\ \ X -'' ill! 



Atn( 



~ 



> ^ 



mx.z 



FIG. 211. THE APPENDAGES OF A CRAB ZO^EA. 



At. I. first antenna ; At. II. second antenna ; nid. mandible (without a palp) ; mx. 

 i. first maxilla; mx. i. second maxilla; mxp. i. first maxilliped ; mxp. 2. second 

 maxilliped. 



ex. exopodite ; en. endopodite. 



times occurs in a very modified form. It makes its appearance 

 in an altered garb in the ontogeny of some of the other groups. 



The two Malacostracan forms, amongst those so far studied, 

 in which the phylogenetic record is most fully preserved in the 

 ontogeny, are Euphausia amongst the Schizopods and Penaeus 

 amongst the Decapods. 



Schizopoda. Euphausia leaves the egg (Metschnikoff, No. 4689) 

 as a true Nauplius with only three pairs of appendages, the two hinder 



