TYPE CRUSTACEA. 



421 



or rather to one in which the pereiopods are indicated but 

 not fully developed, the term Metazoea is applied. Further- 

 more in certain Macrura, such as Scyllarus and Palinurus, the 

 Mysis stage is represented by peculiarly-shaped transparent 

 larvae which have been termed Phyllosoma, or glass-crabs. 

 The carapace is divided into two portions, of which the an- 

 terior or larger covers in the head region and the posterior 

 the thorax, the body being throughout flat and the ab- 

 domen very small. The pereiopods, of which in the earliest 

 stages there are but three, are birarnous, and the first maxil- 



FIG. 194. MEGALOPA-STAGE OF Cancer irroratus (after EMERTON from VERRILL). 



lipeds are either entirely wanting or very rudimentary. Dur- 

 ing successive ecdyses the missing appendages are gradually 

 developed, though the actual transformation of the Pliyllosoma 

 into the youngest Scyllarus or Palinurus stage (which is de- 

 cidedly smaller than the oldest Pliyllosoma) has not yet been 

 observed. 



The change from the Mysis stage to the adult is usually 

 gradual, and no specially definite larval forms are to be found 

 as a rule among the Macrura. In the Brachyura, however, 



