GENERAL NOTES ON CRUSTACEANS 



299 



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

 (Arthrostraca, etc.). 



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. 

 Enphausia and Pencetis, 

 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 in- 

 dications 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 Nauplius becomes the head region of the 

 adult ; the posterior region also persists ; the new growth of 



FIG. 144. Zoaea of common shore crab 

 (Carcinus mcsnas}. After Faxon. 



The appendages are numbered ; f., gills ; 

 ?'., alimentary canal. 



