NO. 3 INSECT HEAD SNODGRASS 8l 



structure owing to the presence of two groups of setae on each (fig. 

 33 B,C). In Nereis vircns, though most of the parapodia are dis- 

 tinctly cleft, those of the first and second segments do not have the 

 double structure (fig. 33 A). Whatever relations, however, may be 

 traced, or assumed to exist, between the annelids and the arthropods, 

 the relationship must be presumed to have come through a remote 

 worm-like ancestor common to both groups, for none of the highly 

 organized modern annelids can be taken to represent the ancestral 

 form of the arthropods. 



A comparative study of the legs of mandibulate arthropods will 

 show that in each group there is a maximum of seven limb segments, 

 l)eyond a subcoxal base, that are individually provided with muscles. 

 The relative size and form of the segments, the character of the articu- 

 lations, and the nature of the musculature present many variations, 

 and it is not to be assumed that segments are to be homologized in 

 all cases by their numerical order beyond the base of the limb. 



The gnathal appendages undoubtedly constitute a group of organs 

 that are individually homologous in arthropod groups, whether their 

 segments are united with the protocephalon to form a larger head, 

 or with the body segments following. The similarity of the structure 

 of the mandible in all the eugnathate arthropods, and the common 

 plan of its musculature, allowing for modifications of which the 

 evolution can easily be followed, leave no doubt concerning the identity 

 of the jaw in the various groups, or that the jaw attained its basic 

 structure in some very remote common ancestor. The primitive struc- 

 ture of the mandible is not entirely preserved in any arthropod : in 

 the Diplopoda and Chilopoda the movable lacinia is retained, but the 

 palpus has been lost ; in the Crustacea and Hexapoda, the lacinia has 

 lost its independent mobility and has become solidly fused with the 

 base of the appendage, but in many crustaceans a mandibular palpus 

 persists. 



The first maxilla of the Hexapoda has the structure of a generalized 

 mandible, i. <?., it consists of a base supporting a palpus and at least 

 one movaTjIe lobe, the lacinia, though generally there is present a 

 second lobe, the galea. The insect labium consists of a pair of ap- 

 pendages that probably once had the structure of the first maxillae. 

 In the Chilopoda the maxillary appendages appear to have under- 

 gone but little modification of structure, and those of the second 

 pair still retain a form similar to that of the body appendages. The 

 corresponding appendages of the Diplopoda are now so highly spe- 

 cialized that it is useless to speculate as to their earlier form. In the 

 Crustacea both pairs of maxillae have been reduced in size and modified 



