June, 19^1.1 Cramptiix: Evoi.utiox of the Anthpopoda. 88 



many tendencies carried over into \vini;:ed insects throufjh the 

 ephemerid and piccopteron types, and since Macliilis is the mosl 

 primitive of these forms, it will serve as the starting point for the 

 next of the series of papers in which it is proposed to trace the modi- 

 fications of the mandihlcs met with in insects in sjeneral. 



Types of Crustaceax Mandip.les. 



Since the Crustacea form the connectinir link lietwcen the lower 

 arthro])ods and the inscctan and myriopodan types, it may he of some 

 interest to note the modifications met with in the more imjjortant 

 groups of Crustacea, wliicli might throw some light u])on tlic condi- 

 tions occurring in the mandibles of insects. The condition met with 

 in the mandibles of the lower forms such as the Copepoda (Fig. 30), 

 Ostracoda (Fig. 33) and P.ranchiopoda (Figs. 22 and 23) is too far 

 removed from the insectan type to be of much interest from this 

 standpoint, especially since the incisor and molar regions are not 

 clearly differentiated in these lower Crustacea. Similarly, the 

 mandible of Ncbalia (Fig. 10) is still too primitive to be of much 

 value in such a study. Mysis (Fig. 7) on the other hand has ap- 

 proached sufficiently close to the type ancestral to insects, to exhibit 

 a number of features such as the differentiation into incisor region 

 proper, gnathofimbrium, and molar region, and the development of a 

 lacinia mobilis, all of which are present in some insects. The 

 mandibular palpus, however, is still very large in proportion to the 

 size of the mandible proper. 



In such Crustacea as Stcgoccplialiis (Fig. 4) there is a pronounced 

 reduction of the palpus "en." while the incisor region " /;/ " becomes 

 slenderer and more elongate, and the gnathofimbrium " c/f " is of 

 greater extent and is composed of seta-like structures forming a hair- 

 like fringe rather than a cluster of spine-like projections as in Mysis 

 (Fig. 7). In the .species of Ascllus shown in Fig. i, the mandibular 

 palpus "en" is quite reduced: but the incisor region "in" is not 

 quite as long as in Stegocephalns (Fig. 4). The gnathofimbrium 

 " gf " is of somewhat less extent than in Fig. 4, and the reduction of 

 the gnathofimbrium " gf " is carried still further in Fig. 3. 



In Diastylis (Fig. 2) the mandibular palpus has completely dis- 

 appeared, and the incisor process is rather .slender and greatly 



