THE METAMORPHOSES OF CRUSTACEA 75 



of the limbs also appear in regular order from before 

 backwards ; the dorsal shield of the nauplius grows 

 out into a carapace, beneath which the paired eyes 

 begin to develop in front. Thus after passing 

 through metanauplius and protozoea stages (Fig. 29, B) 

 the larva becomes a zoea (Fig. 29, C), resembling that 

 of the Crab already described in that the swimming 

 organs are the maxillipeds, but differing in having 

 the uropods well developed and forming a tail-fan at 

 the end of the abdomen, the hinder thoracic somites 

 marked off and their appendages present as rudi- 

 ments, and the stalked eyes free from the carapace. 

 This is followed by a schizopod stage (Fig. 29, D), in 

 which the prawn-like shape is assumed and the 

 thoracic legs have large exopodites used for swim- 

 ming. Later these exopodites diminish in size, 

 though they do not quite disappear in the adult 

 Penceus, and the function of swimming organs is 

 taken over by the abdominal swimmerets. 



In Penceus the larvae are of comparatively simple 

 form, but in the allied genus Sergestes the zoea has 

 a very remarkable appearance. The carapace is 

 armed with long spines, each bearing two comb-like 

 rows of secondary spines. The development of 

 spines and other outgrowths of the surface of the 

 body is a very common characteristic of organisms 

 that, like these larvae, float or swim in the open sea ; 

 its probable significance will be discussed in a later 

 chapter. 



