THE DECAPODA 



303 



spines 



Carcinvs maenas, for instance, the first larval skin lacks the 

 of the Ciirapace, the antennae are larger 

 and diti'erently formed, the shape of 

 the caudal fork and its spine armature 

 are diflferent from those of the succeed- 

 ing stage, and the abdominal portion is 

 not distinctly segmented. These char- 

 acters have been supposed to indicate 

 that we have here the last traces of a 

 Protozoea- stage like that of the Pen- 

 aeidea. 



The Metazoeal stages, which differ 

 from those of the Anomura in the fact 

 that the third maxilliped does not 

 assume a natatory function, are suc- 

 ceeded in nearly all cases by a Mcgalopa- 

 stage (Fig. 182, A, B), in which all the 

 appendages have assumed very much 

 the form which they have in the adult, 

 but the abdomen is large and usually 

 carried extended, and the five pairs of 

 pleopods are used for swimming. In 

 some cases the Megalopa- stage is sup- 

 pressed, the Metazoea being succeeded 

 by a stage in which the animal has 

 assumed the chief characters of the 

 adult. 



Complete suppression of the meta- 

 morphosis occurs in some (perhaps all) 

 Potamonidae, and probably in some 

 other freshwater and terrestrial Brachy- 

 ura. In those Anomura which have 

 become most completely terrestrial {Bir- 

 gtis and Cocnohifa) Borradaile has shown 

 that the young are marine, and that 

 hatching takes place at the Zoea-stage. 



Remarks ox Habit.s, etc. 



The habits and habitats of the 

 Decapoda are more varied, and have 

 been much more studied, than in the 

 case of any other croup of Crustacea. y\('ia-7.opa.oi rorcManahmgicnmu 



^ .,/ ° . ^ , ,1 . (Galatheidea). mf-mf", the tliren 



bpace will not permit of allusion to pairs of maxillipeds : p, rudiments 



more than one or t^vo of the more Ko's^Liilndu'dder^LSdoS.')' 

 salient points. 



