WHAT IS A HEAD ? 33 



and anotlier. But the joints themselves are sometimes 

 consolidated, and 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 thos3 in front of the month- 

 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 number, we find them successively devoted 

 to seeing, feeling and otherwise perceiving, feeding and 

 presumably tasting, 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 



