Carpenter — EekitionshijJS between C/asses of Arthropoda. 34'J 



pairs of limbs, was preceded by a head with four pairs, and that 

 the far-off ancestors of the Arachnida had a head -with five jjairs of 

 limbs, the foremost of which were feelers. (See Table, pp. 354-5.) 



Belationship between Insects and Crustaceans. 



Belief in the multiple origin of the Arthropoda rests chiefly on a 

 supposed radical divergence between Insects and Crustaceans.^ It has 

 been shown in the previous chapters of this essay that^Insects, as well 

 as Centipedes and Millipedes, can be traced back to ancestral Arthro- 

 pods with five limb-bearing head- segments, and fifteen Kmb-bearing 

 trunk- segments ; and that Crustaceans and Arachnids can be traced back 

 to an ancestral stock showing precisely similar segmentation. It has 

 been further pointed out that the identity of segmentation in three 

 distinct classes cannot be reasonably explained as the result of con- 

 vergence. The strongest presumption is raised for a real kinship 

 between Insects and Crustaceans. It is desirable, therefore, to compare 

 the two classes in some points of detail with the view of more firmly 

 establishing their relationship. 



General agreement now exists among zoologists that the feelers, 

 tritocerebral vestiges, and mandibles of Insects represent^ respectively 

 the antennules, antennae, and mandibles of Crustaceans. Both the 

 general form of the appendages and the ganglia from which they 

 are innervated afford evidence that this view is correct ; and Hansen 

 ('93) has brought forward facts that tell most strongly in support 

 of the homology of the Crustacean with the Insectan mandibles. 

 He points out that there is a very close likeness, both in form and 

 musculature, between those jaws in the Thysanui'a and Collembola 

 on the one hand, and the lower Malacostraca, especially the 

 Cumacea, on the other. Indeed there is a much closer likeness between 

 the mandibles of Diastylis and those of Machilis than between the latter 

 and the mandibles of Blatta or any winged Insect. 



Most writers on Arthropod morphology have not hesitated to range 

 the two pairs of Crustacean maxillae with the similarly-named appen- 

 dages in Insects. But, as we have ali-eady seen, the researches of 

 Hansen ('93) and Folsom ('00), supported by the facts brought forward 

 early in this essay (pp. 324-5), compel us to recognise the maxillula) 

 of the Thysanura as a pair of jaws anterior to the two pairs of 

 maxillae. Hansen has further called attention to the points of like- 

 ness in detail between them and the first maxillae of Crustaceans, and 

 between the first maxillte of Insects and the second maxillae of Crus- 



