NO. 3 INSECT HEAL) SNODGRASS 79 



It will be shown in the next section of this paper that the cardines 

 of the maxillae are not true proximal segments of the maxillary 

 appendages, but are secondary subdivisions of the bases of these 

 appendages. It appears probable, therefore, that the submentum 

 represents likewise proximal subdivisions of the bases of the second 

 maxillae, retaining the lateral articulations with the margins of the 

 cranium in generalized insects (fig. 36C, /), and perhaps including 

 between them a median part of the labial sternum. 



If the insect labium (figs.< 32 A, 40 D) is compared with the second 

 maxillae of a chilopod (fig. 32 B), it will be seen that the united basal 

 segments of the latter {Cx), containing the origins of the palpal 

 muscles {O, Q) , correspond at least with the mentum of the labium. 

 The large proximal segments of the chilopod maxillae are clearly the 

 bases of a generalized limb, the coxae, according to Heymons (1901), 

 and the limb base, or a subcoxal division of it, bears the primitive 

 dorsal articulation of the appendage with the body. The mentum, 

 and at least the lateral parts of the submentum, therefore, appear to 

 be subdivisions of the primary bases of the second maxillary ap- 

 pendages, corresponding with the stipites and cardines of the first 

 maxillae in insects, and with the similar subdivisions of the bases of 

 the mandibles in the diplopods (fig. 26 A, Cd, St). 



The median, terminal duct of the labial, or " salivary," glands opens 

 anterior to the labium, and, in typical forms, at the base of the mentum 

 (figs. 18 D, 19, SIO). The position of the orifice, anterior to the sub- 

 mentum, however, does not argue that the latter is entirely the sternum 

 of the labial segment, but rather the reverse, for it is likely that the 

 orifice of the duct has not left the sternal region of its segment, and 

 that it has been crowded forward in the latter by the median approach 

 of the labial cardines. The common duct of the lal)ial glands results 

 during embryonic development from the union of the two primary 

 ducts of paired lateral glands of the labial segment. 



MORPHOLOGY OF THE GNATHAL APPENDAGES 



It has often been assumed that the segmental appendages of all 

 arthropods are derived from a primitive limb having a biramous type 

 of structure. A two-branched limb, however, occurs actually only in 

 the Crustacea, and there is no certain evidence of a biramous limb 

 structure ever having prevailed in other arthropod groups. In all 

 forms, including the Crustacea, the segmental appendages first appear 

 in the embryo as simple protuberances of the body wall, and some 

 zoologists now believe that the exopoditc branch, when present, is 



