420 INVERTEBRATE MORPHOLOGY. 



thoracic appendages in addition to those already present in 

 the Nauplius, and furthermore by the distinct separation of 

 the body into an anterior cephalo-thoracic portion covered by 

 a carapace and a posterior abdomen which is usually but 

 imperfectly segmented. This stage is succeeded sometimes 

 after two or more ecdyses by the Zo'ca (Fig. 192, ), a stage in 

 which the majority of Decapoda leave the egg. It is distin- 

 guished from the Protozoea principally by the perfect segmen- 

 tation of the abdominal region, though it still possesses no 

 appendages, unless it be rudiments of the sixth pair, and it 

 is furthermore characterized by the compound eyes being 

 stalked, a feature but slightly indicated in the Protozoea, in 

 which stage they make their appearance. The Zoea stage in 

 the Brachyura is generally characterized by the development 

 of spines, sometimes of enormous length (Porcellana), upon the 

 dorsuin and sides of the carapace. 



In such a form as Euphausia the next stage is the adult, 

 but in the Decapods other larval stages intervene before the 

 adult condition is reached. The first of these is characterized 

 in the majority of the Macrura by the appearance of the re- 

 maining thoracic appendages which were unrepresented in 

 the Zoea, in the form of biramous structures closely resem- 

 bling the thoracic appendages of the Schizopoda, whence the 

 stage is generally termed the Mysis stage (Fig. 193). The 



FIG. 198. - MYSIS STAGE OP LOBSTER, Homarus americanus (after S. I. SMITH). 



abdominal appendages also develop during this stage. 

 Among the Hermit-crabs (Paguridae) and the Brachyura the 

 development is to a certain extent abbreviated, the pereiopods 

 never being represented by biramous appendages, but being 

 from the first uniramous, and in these forms therefore a 

 true Mysis stage never occurs. To the corresponding stage, 



