318 PHYLUM ARTHROPODA. 



In Arthropods one or more of the anterior appendages is 

 modified to form jaws (foot- jaws), and in association with this 

 we find a concentration and fusion of some of the anterior seg- 

 ments of the body by which they form a group distinct from 

 the segments behind and constitute a head. 



From the study of development it is clear that, as pointed 

 out by Lankester, there occurs in Arthropods a shifting of the 

 position of the mouth backwards, in relation to the segments 

 of the head ; so that it may come to lie in the adult on the 

 ventral side of the segment which is apparently * the second, 

 third or fourth. The first pair of appendages having the 

 character of jaws lies either at the sides of or immediately behind 

 the mouth, and hence in the several groups the jaws are the 

 appendages of the apparent second, third or fourth segment, 

 the appendages anterior to them taking on other and generally 

 tactile functions or in some cases disappearing altogether. 

 Bearing in mind the fact that in Peripatus the blastopore ex- 

 tends, at one stage in development, as a fissure along the ventral 

 median line between mouth and anus, it may without difficulty 

 be conceived how such an alteration in the position of the mouth 

 may have occurred. 



The segmentation of the arthropod head. It would be inter- 

 esting, if it were possible, to determine the relation between the 

 anterior segments of the body which in the several divisions of 

 Arthropods form the head and also between them and the 

 anterior segments of the Annelida. 



The post-cephalic segments are very variable in number in 

 both groups. The body is divided up, here into a larger, there 

 into a smaller number of segments, and the problem of the 

 precise homology of a particular segment in one class with 

 the numerically corresponding segment in another is probably 

 without reality. 



With regard to the head however the case appears to be 

 different. Within the divisions of the Arthropoda (Insecta, 

 Crustacea, etc.) the number of head-segments shows a remarkable 

 constancy ; the segmentation has become, as it were, numeric- 

 ally stereotyped, and we may inquire how far the segments 

 correspond in the several divisions, and whether a numeri- 



* The difficulty of determining which segment of an Arthropod corre- 

 sponds with the first in the Annelida is pointed out below. 



