THE DECAPODA 



271 



p. 260) and the Brachyura. In most Anomura the last pair, and in 

 a few Brachyura the last or the last two pairs, are subchelate. A 

 very remarkable form of chela is found in the genus Psalidopus 

 (Caridea) (Fig. 163), in which 

 loth fingers are movably articul- 

 ated with the propodite, an 

 arrangement resembling that 

 found in the second maxilliped 

 of Stylodactyfa*. 



In most of the Reptantia, 

 where the first pair of legs are 

 chelate and much larger than 

 the others, they are commonly 

 referred to as the chelipeds, and 

 the following four pairs are 

 distinguished as walking -legs. 

 Frequently the chelipeds are 

 asymmetrical in size and shape 

 on the two sides, the larger 

 chela having the fingers armed 

 with blunt crushing -tubercles, 

 while the smaller has sharp 

 cutting -teeth. In many cases, 

 as, for instance, in the lobster, 

 the larger crushing -chela may 

 be on the right or the left side 

 indifferently, but in some Bra- 

 chyura it is constantly on the 

 same side of the body. A 

 curious reversal of asymmetry 

 sometimes occurs as a result of 

 the loss of the larger chela ; at 

 the next ecdysis the remaining 

 chela assumes more or less com- 

 pletely the characters of a large 

 crushing- chela, while the re- 

 generating limb has the form 

 of a small cutting-chela. 



A modification Of SOnie Of . ftHftwJete Uextii (Bryonidea). The vesti- 



. gial eye-stalks are fixed in notches in the front 



the legs as SWimming -paddles of the carapace. (From Alcock, Naturalist in 



occurs in various groups, for in- 

 stance, in the Portunidae (Brachyura), where the last pair are so 

 modified. In some Natantia and in one genus of Hippidea one 

 pair of legs may become multiarticulate and flagelliform. This 

 modification occurs especially in the second pair of many Caridea 

 (formerly grouped together as Polycarpinea) (Fig. 148), where the 



