3S8 



PHYLUM ARTHROPODA 



Compared with Astacus, however, the most important 

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

 striking metamorphosis in the hfe-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 pro- 

 found changes. Tn some types {Nebalia, My sis) a meta- 

 morphosis takes place 

 within the egg-case, and 

 in the few forms in which 

 development seems to be 

 direct, slight traces of 

 metamorphosis are found. 

 Almost all the lower 

 Crustaceans and some 

 higher forms, e.g. Eu- 

 phaiisia and Penceus, are 

 hatched in a Nauphus 

 stage. In the remaining 

 types the Nauplius stage 

 is indicated within the 

 egg by the moulting of 

 a larval cuticle (as in 

 Astacus). The Nauplius 

 is characterised by a 

 typically rounded body, 

 and by the presence of 

 three pairs of appendages, 

 which are the only ob- 

 vious indications of seg- 

 mentation. The first pair 

 of appendages are un- 

 branched, and bear larval 

 sense organs, the next 

 two are biramose swim- 

 ming organs. There is an unpaired median eye, but no 

 heart, and frequently no hind-gut. The three pairs of ap- 

 pendages 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 segments and appendages takes 

 place (with numerous moultings) in the region between these. 



Fig. 20I. — Zoasa of common shore- 

 crab {Carcinus mcenas). — After 

 Faxon. 



The appendages are numbered ; c. 

 i., alimentary canal. 



gills ; 



