WHAT IS A HEAD? 30 
and another. But the joints themselves are sometimes 
consolidated, atid then it is said that two or more segments 
have coalesced. ‘This is not a merely arbitrary statement, 
for, comparing the Malacostraca one with another, the 
conclusion cannot be avoided that a single segment is 
limited to a single pair of appendages. Segments which 
are not independent in one of the families will be found 
well articulated in another, and those which can least 
boast of freedom nevertheless frequently point to an origi- 
nal independence by some suture or groove, if not by the 
actual separateness of the segmental ring in some small 
part of its circuit. 
No rigid definition is possible of a head. It is bound 
to contain the animal’s mouth, and may be expected to 
include the brain and organs of the senses of sight and 
hearing, smell and taste. In birds and in mammals its 
limits are conveniently defined by the neck, but in the 
Crustacea there is no such obvious constriction separating 
it from the trunk. Consequently its true limits here are 
still a subject of dispute, which cannot be settled offhand 
by an appeal to the cervical groove, even when that is con- 
spicuous. By various authorities the first five, six, or 
seven segments have been assigned to the head, and in the 
higher crustacea it might not unreasonably be regarded as 
comprehending the first nine. This will be understood 
from a consideration of the form and functions assumed by 
the several appendages, only thos» in front of the mouth- 
opening or directly contiguous to it being accepted without 
dispute as cephalic, although others in variable number 
are concerned in the operation of feeding. 
Glancing along the whole line of limbs, as the out- 
growths from the segments have some right to be called, 
twenty pairs in uumber, we find them successively devoted 
to seeing, feeling and otherwise perceiving, feeding and 
presumably tasting g, grasping and striking, walking and 
digging, swimming and leaping. But although the order 
in which they act may thus be generally stated, there is 
not unfrequently a transfer of function from one part of 
the line to another. The feelers may be employed to assist 
