THE ARTHROPODA 



99 



and flabellum are borne on the outer margin of the third 

 segment of the axis; they are reduced in size, but do not 

 present any other remarkable features. The whole limb, with 

 its jointed axis and polyarthrous filamentous endites, presents 

 many resemblances to the jointed limbs of higher Crustacea, 

 and it is interesting to observe the gradual transitional stages 

 between it and the more posterior lobed foliaceous limbs 

 characteristic of the order Phyllopoda to which Apus belongs. 



The three pairs of appendages in front of the first thoracic 

 limb are small in size, and lie close round the mouth. It 

 will be convenient to begin with the most anterior of the 

 three pairs viz. the mandibles. These are short, stout, 

 curved structures with thick chitinous walls. Their long axes 

 lie nearly transversely to the body, and their inner edges, 

 which meet below the mouth, are furnished with a number 

 of stout conical teeth. They are moved by very powerful 

 muscles, and are the functional masticatory organs of the 

 animal. The mandible represents the basal part of the axis 

 of an ordinary limb, the terminal part with the endites and 

 exites having disappeared. 



The second pair of oral appendages are called the first 

 maxillae. They are small, and each consists of two pieces 

 fitted together in a peculiar manner. The hinder of the two 

 pieces has the shape of a gnathobase, and probably represents 

 that organ. The anterior piece is a half arch of chitinous 

 substance, expanded at its distal end into a broad lobe with 

 a setose margin. The convexity of the arch forms a smooth 

 articular surface against which the posterior piece works. It 

 is not clear what part of a typical appendage is represented 

 by this anterior piece, but it is certain that the two pieces 

 are parts of one limb, and are not two separate limbs as some 

 authors have supposed. The third pair of oral appendages 

 are sometimes called the maxillipedes, but it is better to call 

 them the second maxillae. They are very small and rudi- 

 mentary, each member of the pair consisting of two lobes 

 united by a low ridge. The inner lobe is provided with 

 marginal setae, and probably represents the gnathobase, which 

 it resembles more nearly than any other part of a complete 

 limb. The outer lobe is cylindrical, and devoid of setae : it 

 is to be regarded as the representative of the bract, but 

 the most important thing about it is that the duct of the 



