72 Journal New York Entomological Society. C^'o'- xxix. 



("ex" of textfigure i, and Fig. 9) of the limb doubtless served as a 

 gill — or it may even have aided in locomotion as well. The probable 

 paths of development followed in the evolution of the different types 

 of mandibles of interest from the standpoint of the development of 

 the insectan type, have been sketched below. 



Evolution of the Mandibles. 



As was mentioned above, the trilobitan type of biramous mandi- 

 bular appendage, in which the exopodite "ex" (textfigure i) is still 

 retained, and the endopodite '' en " still functions in locomotion, while 

 the basal segment (like that of the body limbs as well) has become 

 modified in a fashion which enables it to function in the holding and 

 comminuting of food, serves as a convenient starting point in tracing 

 the series of modifications leading to the production of the insectan 

 type of mandible. The mesal region of the basal segment of the limb 

 is producted to form the so-called gnathobase " gb" (textfigure i, and 

 Fig. 9 of Plate VI) which abuts against its fellow projecting from 

 the limb of the opposite side of the body, and serves to manipulate 

 and comminute the food (which was probably of the nature of soft 

 bodied worms or similar creatures, as is the case with Limulus and 

 related forms living today). The gnathobase is provided with stout 

 spine-like projections which doubtless aided in comminuting the food, 

 and I think that a portion of the surface bearing these projections 

 becomes involved in the composition of the incisor region of the 

 mandibles of higher arthropods. It is possible that the endite "gb" 

 of the basal segment of the limb of Apiis, shown in Fig. 20 (Plate 

 VII) corresponds to the gnathobase " gb" of the trilobite shown in 

 Fig. 18 (or in textfigure i) ; but the other endites " ei" of the limb 

 of Apiis (Figs. 20 and 19) appear to be articulated appendages of 

 the limb differing slightly from the gnathobase " gb " which is a 

 prolongation of the entire mesal region of the basal segment of the 

 limb in the trilobite shown in Fig. 18, for example. 



A somewhat higher stage of development is represented by the 

 condition exhibited by a limb of the merostome Limulus shown in 

 textfigure 2 (compare with Fig. 8, Plate VI). As m.ay be seen in 

 textfigure 2, there is a tendency to lose the exopodite ("ex" of 

 textfigure i) in the limb of a merostome. while the point of attach- 



