4.0 A HISTORY OF RECENT CRUSTACEA 
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 XIT., 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 cesopha- 
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. 
