314 PHYLUM ARTHROPODA. 
an invagination of the simplest embolic type (Zucifer), and 
through the condition described for Asfacus, 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 (WWVebalia, 
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 Peneus, 
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 
Fic. 166.—Zozea ot common shore-crab 
(Carcinus menas).—After Faxon. rounded. Dad, and. by 
a pakeacy the presence of three 
e appendages are numbered ; ¢., gills; : 
z., alimentary canal, pairs of appendages, 
F 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 antenne and 
the mandibles of the adult. The head region of the 
