A2 A HISTORY OF RECENT CRUSTACEA 
7. The appendages of the seventh segment are dis- 
tinguished from the two preceding pairs by the name 
maxillipeds, meaning maxillary feet or jaw-feet, because 
they often conspicuously combine the function of a mouth- 
organ with the general appearance of a crustacean leg. 
This is very much the case in the Amphipoda, where they 
conclude the series of cephalic appendages, having here 
something of the opercular function which they exercise 
still more conspicuously in the Isopoda. In the Copepoda, 
which are content with one pair of maxille, there are two 
pairs of ‘ foot-jaws, in regard to which the singular dis- 
covery has been made that they belong to a single segment, 
and yet do not violate the rule that a single segment is 
limited to a single pair of appendages. By tracing the 
process of development Dr. Claus made it clear that they 
were in fact the exopods and endopods of a single pair of 
limbs, which had separated so as to wear the appearance 
of two distinct pairs, which is much as if the radius and 
ulna in the human arm were to become independent and 
produce a four-handed man. 
In the higher Crustacea these appendages, without 
being divided, are only the first of a series of three pairs 
of maxillipeds. ‘Their forms vary exceedingly in the 
different groups. Sometimes the endopod is seven-jointed 
like an ordinary limb, at others the terminal part is 
reduced to insignificance. Often the epipod and exopod 
are important both in size and function. 
8. In regard to the eighth segment, a difficulty arises 
as to how the appendages should best be designated. In 
some groups, such as the Amphipoda and Isopoda, they 
belong not only in theory but in fact to the trunk, and 
they have in these groups been called gnathopods, a word 
which, like maxillipeds, means jaw feet, and which was 
chosen to indicate that they grasped the food in a jaw-like 
manner. But in the h'gher Crustacea these appendages 
practically belong to the mouth and not to the trunk, 
their general appearance and functions allying them 
closely to the preceding pair. Under these circumstances 
it seems best to call them the second maxillipeds. For, 
