40 A HISTORY OF RECENT CEUSTACEA 



size, it is a question whether the trunk of the mandible 

 may not represent a coalescence of the first and second 

 joints, or even of the first three or four joints. The latter 

 supposition would explain the circumstance above noticed 

 that its ' palp ' or terminal portion in the Malacostraca 

 never exceeds the number of three joints, though it may 

 be reduced to two or one, or may vanish altogether (see 

 Plates XII., XV., XIX.). On the inner side of the man- 

 dible there is sometimes, adjoining the cutting edge, a 

 plate which more or less closely imitates that edge in its 

 character. It has now been made probable that this 

 secondary plate is a modification of one of the spines above 

 noticed, and, as if to show the plasticity of nature, some- 

 times the series of spines mimic the secondary plate. 



The upper and lower lip seem best to be regarded as 

 modifications of the integument, where it is turned in to 

 form the alimentary canal, commencing with the oesopha- 

 gus or gullet. 



An American lady, Miss J. M. Arms, however, in her 

 very clever little work on Crustacea, in Alpheus Hyatt's 

 ' Guides for Science-teaching,' maintains that the leaves of 

 the lower lip ' are independent outgrowths or buds from 

 the integument, as much as any other pair of appendages; 

 and the fact that the parts of the segment to which they 

 must have belonged have disappeared, or cannot be readily 

 found, is,' in her opinion, ' an argument of doubtful 

 weight.' 



The theory that all the appendages of a crustacean are 

 either legs or modified legs will strike a casual observer 

 as rather strained in its application to the mandibles. 

 That a crab should adapt the basal joints of a pair of 

 limbs for masticating its food may seem as unlikely and 

 absurd as that a man should have teeth on his elbows, 

 and should draw them up in front of his lips for the 

 purpose of biting and chewing whatever he wished to put 

 into his mouth. To prevent all cavilling, however, on 

 this point of the theory, the King Crab, Limulus, is so 

 obliging as to ignore the ordinary mouth organs, and to 

 use the bases of its actual walking legs as mandibles. 



