1920 ENTOMOLOGICAL SOCIETY. 109 



" hereditary area " labeled " X " in Fig. 1, would naturally be expected to resemble 

 the Crustacea quite closely, since the territory which they occupy is contiguous 

 to that of the higher Crustacea. Similarly, those insects which occupy the 

 " hereditary area " labeled " Y," would greatly resemble the " Myriopoda," since 

 the territory which they occupy is contiguous to that of the " Myriopoda." Cross 

 sections of the three lines of descent at the level " B " would be represented as 

 three intersecting circles (Fig. 3), each of which, taken separately, demarks a 

 distinct group (Crustacea, Insecta and Myriopoda) ; but the intersecting circles 

 have a certain amount of territory in common, and those insects in the area labeled 

 " X " (Fig. 3) being next to the Crustacea, would naturally have much in common 

 with the Crustacea (left hand circle), whilg those insects in the area labeled " Y" " 

 being next to the "Myriopoda" (right hand circle) would naturally have much 

 in common with the "Myriopoda." If we trace the lines of descent back to the 

 level "C" (Fig. 1) they are seen to merge in a common " crustaceoid " ancestry; 

 and a cross section at this level would represent the circles as completely coinciding 

 (Fig. 4). It is thus readily comprehensible that there may be a dual relationship 

 between the Insecta and higher Crustacea, on the one hand, and between the In- 

 secta and the " Myriopoda " on the other — as we are forced to conclude is the case, 

 from a study of the anatomy and embryology of the forms in question. This 

 may indicate that the group Insecta is a polyphyletic one, and although I have 

 been loath to accept this view, I can see no escape from the conclusion that insects 

 are very closely related to both the higher Crustacea (Isopoda, Tanaidacea, 

 Cumacea, etc.) and the " Myriopoda." 



Since it is quite evident that the lines of descent of the higher Crustacea, 

 Insecta, and " Myriopoda " soon merge in a common ancestry, the question natur- 

 ally arises as to what these common ancestors were like. That these common 

 ancestors were all of one type is out of the question, for they apparently differed 

 among themselves as much as the Mysidacea, Anaspidacea and other " intermediate 

 Malacostraca " (possibly including Arthropleura also) differ among themselves; 

 and these common ancestors probably resembled all of the forms just mentioned 

 (i.e. the Mysidacea, Anaspidacea, etc.), though it is possible that the Cumacea and 

 Tanaidacea are more like the immediate ancestors of insects than are the Mysidacea, 

 Anaspidacea, etc., which are more like their remote ancestors. 



The Anaspidacea, Mysidacea, and other " intermediate Malacostraca " are in 

 turn derived from ancestors resembling the Nabaliacea and other primitive 

 Malacostraca, and the lines of development of the malacostracan Crustacea have 

 undoubtedly accompanied those of the insects and " myriopods " more closely and 

 for a longer distance than any other forms have done. The primitive malacostracan 

 Crustacea such as Nehalia and its allies, exhibit undoubted affinities with the 

 Branchiopoda and Copepoda, and to some extent with the Trilobita also, and they 

 have even preserved some ancestral features in common with the Merostomata, 

 although the latter forms lead off toward the lines of development of the 

 Arachnoidea, and away from the lines of development of the higher Crustacea, 

 Insecta, and " Myriopoda." 



The question as to which arthropods have departed the least from the common 

 ancestors of the phylum Arthropoda is an extremely difficult one to answer. The 

 Copepoda, Branchiopoda and Trilobita are among the most primitive known 

 arthropods, and it is quite probable that the first representatives of the group 

 combined in themselves characters common to all three. Thus, for example, the 

 earliest arthropods were in all probability not trilobites alone, but were doubtless 



