474 GEORGE H. CARPENTER. 
which rests the existence of each somite, not evidently 
present in the various animals. 
The conclusion to be drawn from this numerical corre- 
spondence in segmentation between typical members of the 
leading Arthropod classes is not merely that there must have 
existed common ancestors of all, with distinctively arthro- 
podous characters, but that these ancestors must have 
possessed the definite number of segments still traceable in 
their descendants. It follows from this that Arthropods with 
very numerous segments—Apus, Julus, or Geophilus, for 
example—must be regarded as, in that respect at least, 
specialised forms. “ Rich segmentation” has been so freely 
assumed to: be necessarily a primitive character, that my 
view will probably not find ready acceptance with many 
students. For the present I simply lay stress on the fact that 
the Symphyla (which combine so strikingly the characters of 
Insects, Centipedes and Millipedes) and Polyxenus (which 
belongs to the primitive diplopodan order Pselaphognatha) 
both exhibit the typical and definite number of segments 
characterising “a Lobster, a Cockroach, and a Scorpion.” 
Kinship between Insects and Crustaceans. 
Perhaps the point which needs strongest enforcement in 
order to recall zoologists to a reasonable view of the Arthro- 
poda as one phylum is the somewhat close relationship that 
exists between Insects and Crustaceans. I have already (3, 
pp- 343-7) discussed this relationship at some length, and the 
arguments in favour of it are strongly pressed in Lankester’s 
article (19, p. 573). The existence in the Collembola and 
Thysanura of maxillule whose true nature as a pair of appen- 
dages between the mandibles and first maxilla has been 
established by Hansen (6) and Folsom (4), shows the closest 
correspondence between the Insectan and the Crustacean 
head, with which the foremost trunk-segment undergoes more 
or less complete fusion, its appendages forming the first 
maxillipeds in the Malacostraca and the second maxille 
