94 Journal Xew York Entomological Society, t^'o'- >^>^ix. 



lines of descent of the higher Crustacea appear to parallel that of the 

 insects for a much longer distance, even until the lower forms of 

 winged insects were developed, since many crustacean characters are 

 carried over into the lower types of winged insects such as the 

 Ephemerida, etc. The study of the anatomical features of the Crus- 

 tacea is therefore of much greater importance than that of the 

 " myriopods." and in the foregoing discussion, it has been shown 

 that at least one type of insectan mandible has been derived more 

 or less directly from a type occurring in the Crustacea. The deriva- 

 tion of the other types of mandibles occurring in insects will be dis- 

 cussed in a second paper of this series, in which it is proposed to 

 take up the evolution of the modifications met with in the different 

 orders of insects as well. 



Summary. 



The principal points brought out in the foregoing discussion may 

 be briefly summarized as follows : 



1. The parapodium of an annelid represents the probable pre- 

 cursor of the primitive biramous arthropodan limb, which in turn is 

 the forerunner of the mandibular appendage of Crustacea, Insecta, 

 and " Myriopoda." 



2. Insects, myriopods, and Crustacea form a subphylum (the 

 Eugnathata) in which the mandibular appendage is essentially jaw- 

 like rather than limb-like. In the rest of the Arthropoda (which con- 

 stitute the subphylum Podognathata) the limbs homologous with the 

 mandibular appendage are not jaw-like, and trilobites appear to be 

 slightly nearer the latter group than the former, although they are 

 anatomically intermediate between the two groups. 



3. The biramous mandibular appendage of trilobites, in which 

 both exopodite and endopodite are retained, and in which the basal 

 segment of the limb has become slightly modified for holding food 

 through the development of a gnathobase, while the rest of the 

 appendage still serves a locomotor limb, forms the starting point for 

 tracing the modifications met with in the mandibular appendage of 

 Crustacea, Insecta and " Myriopoda." 



4. The first steps in the production of a true mandible from such 

 an appendage are the loss of the exopodite, and the reduction of the 

 endopodite to a mandibular palpus, accompanied by the greater de- 



