PHYLUM ARTHROPODA. 



an invagination of the simplest embolic type (Ludfer\ and 

 through the condition described for Astacus, to the forma- 

 tion of endoderm by the ingrowth of a solid plug of cells. 

 Compared with Astacus, however, the most important 

 point we have to notice is the frequent occurrence of a very 

 striking metamorphosis in the life history. In other words, 

 the larva hatched from the egg is rarely like the parent, and 



only acquires the adult 

 characters after a series 

 of profound changes. In 

 some cases (Nebalia, 

 Mysis) a metamorphosis 

 takes place within the 

 egg-cases, and in the few 

 forms in which develop- 

 ment seems to be direct, 

 slight traces of meta- 

 morphosis are found. 



Almost all the lower 

 Crustaceans and some 

 higher forms, e.g. 

 Euphausia and Penceus^ 

 are hatched in a Nauplius 

 stage. In the remaining 

 cases the Nauplius stage 

 is indicated within the egg 

 by the moulting of a larval 

 cuticle (as in Astacus). 

 The Nauplius is char- 

 acterised by a typically 

 rounded body, and by 

 the presence of three 

 pairs of appendages, 

 which are the only 

 obvious indications of segmentation. The first pair 

 of appendages are unbranched, and bear larval sense 

 organs, the next two are biramose swimming organs. 

 There is an unpaired median eye, but no heart, and 

 frequently no hind-gut. The three pairs of appendages 

 become the first and second pairs of antennae and 

 the mandibles of the adult. The head region of the 



FIG. i66. ; Zoseaof common shore-crab 

 (Cardnus manas}. After Faxon. 



The appendages are numbered ; c., gills ; 

 /., alimentary canal. 



