TYPE CRUSTACEA. 



419 



all the members of the class, and it is only in the Cirrhipedia 

 that a second definite larval form can be distinguished, the 

 Cjpris-larva, to which attention has already been called (p. 399). 

 In the Malacostraca the occurrence of a free-swimming 

 Nauplius is the exception rather than the rule, and indeed 

 larval forms are practically wanting in some groups, such as 

 the Leptostraca and Arthrostraca, and in certain species or 

 families of other groups (e.g. Mysidce, Gambarus). In the 

 genus Penceus among the Decapods, and in Euphausia among 

 the Schizopods, a typical free-swimming Nauplius occurs, and 

 in Lucifer the embryo leaves the egg in the form of the Meta- 

 nauplius. In the majority of forms these stages are passed 

 over while the embryo is still within the egg-shell, and it 

 hatches only when it has acquired a greater degree of develop- 

 ment. In such forms as Penceus, Eupliausia, and Lucifer the 

 Metanauplius stages pass into what is termed the Protozoea 

 (Fig. 192, A) a stage also passed over within the egg by the 



FIG. 192. A, PROTOZOKA OF Lucifer (after BROOKS); B, ZOEA OF Palcemonetes 



(after FAXON). 



At 1 = autenuule. m = mandible. 



Af 2 = antenna. mx l , mx* = maxillae. 



c cerebral ganglion. mp 1 , mp* = maxillipeds. 



E = compound eye. oc = simple eye. 



h = heart. r = rostrum. 



s = stomach. 



majority of Malacostracaus, though occurring as the first 

 larval stage of some Stomatopods. It is characterized by the 

 development of two maxillse and the two or three anterior 



