Crustaceans. 



43 



in the female than in the male, in which it is very narrow and 

 with the swmmerets much reduced in number. The majority 

 of marine Decapods are not hatched from tlie egg in the form 

 of the adult, but pass through a number of larval stages, which 

 are of great interest as affording, in many cases, a clue to the 

 ancestrj' of the different forms (Fig. 45). These larv£e vary 

 considerably accordmg to the genera, but the following short 

 account of the development of the common shore crab will give a 

 general idea of the transformations to which a Crustacean is 

 subject in the course of its life. 

 The young leaves the egg as a 

 tiny transparent larva, just visible 

 under a strong lens. From this 



FIG. 45. LARV.E OF CARCINUS MOEN.'^S (AFTER BELL). 



early stage the crustacean characters are already apparent, 

 carapace and abdomen are weU developed, but the eyes are 

 sessile, not stalked or pedunculated, and the walking-legs and 

 swimmerets are absent, locomotion being effected by the more 

 anterior appendages which, later in hfe, function as mouth 

 parts. This little larva, known as Zoiia, swims about the surface 

 of the sea, feeding voraciously and undergoing a series of moults, 

 in the course of which the hinder appendages of the carapace and 

 abdomen gradually appear. Later on the eyes acquire stalks 

 and the carapace broadens, the larva reaching the stage known 



