THE DECAPOD A 



259 



is modified in a very peculiar manner, growing over and, in most 

 cases, completely enclosing the eyes. 



The differences in the development of the abdominal region are 

 no less conspicuous than in the case of the cephalothorax, and 

 have been utilised as affording characters for the primary sub- 

 divisions of the Order. In the Natantia the abdomen is large and, 

 with its appendages, forms the chief organ of swimming. It is 

 generally more or less compressed and its somites have well- 

 developed pleura. It is dorsally humped or bent between the 

 third and fourth somites in many Caridea (Eukyphotes, Boas) (Fig. 

 148), but the character is not so constant as to justify great 



Fio. 148. 



Hetei'ocarpus Alphonsi (Caridea, Pandalidae), showing the "humped" form of the abdomen 

 and the multiarticulate meropodite and carpopodite of the second leg. (From Alcock, 



Xiititmlixt in Indian Seas.) 



systematic importance being attached to it. In the Palinura and 

 Astacura the importance of the abdominal appendages as natatory 

 organs is generally reduced, and the abdomen itself is not humped. 

 Among the Anomura, the Thalassinidea (Fig. 149) have retained 

 the extended abdomen of the Macrurous groups, but the pleura 

 are more or less reduced; the Galatheidea (Fig. 150) have the 

 abdomen more or less closely flexed under the cephalothorax but 

 not greatly modified; the Paguridea, with the exception of some 

 interesting transitional forms (Pylochelidae) (Fig. 151), have the 

 abdomen and its appendages more or less unsymmetrically developed 

 and its somites imperfectly indicated. In the hermit-crabs (Pagu- 

 ridae and Coenobitidae) the abdomen is soft-skinned and spirally 



