Carpenter — Relationships between Classes of Arthvopoda. 345 



the Insects and their allies actually branched off from the Crustaceans ? 

 Or is it more reasonable to regard Insects and Crustaceans as divergent 

 stems from a common far-off Arthropod ancestry ? 



With regard to this question, the conclusions of Hansen incline 

 towards the former view. Although (Hutton, etc., '97) he " dislikes 

 ancestor-hunting and pedigrees," he expresses the opinion that "the 

 lower Malacostracous Crustacea and the Thysanura are more closely 

 related to each other than hitherto recognised." And Lankester ('81) 

 admitted the possibility of deriving the Insecta from the Isopoda, 

 although more recently ('97) he has suggested a wide divergence 

 between all Insects and all Crustaceans. As we have already seen, 

 there is a close agreement in the segmentation of both head and body 

 in the two groups. When we find further that there is a striking 

 similarity between the Thysanuran and Crustacean mandibles, that 

 Insects agree with Amphopods and Isopods in possessing sessile com- 

 pound eyes, and that the minute structui'e of these eyes in certain 

 Thysanura, as shown by Oudemans ('88), agrees with that of the 

 Crustacean eye in the presence of a hypodermal layer, wanting in the 

 higher Insects, between the corneal facets and the crystalline cones, it 

 must be at least admitted that the origin of Insects from the lower 

 Malacostraca is worthy of discussion. To imagine a close connexion 

 between Insects and Isopods is easy on account of the adaptation to a 

 terrestrial life shown by the Oniscidae. But in Isopods, the hinder pair 

 of feelers are strongly developed ; while in Insects these appendages 

 are represented only by embryonic rudiments ; and the Thysanuran 

 mandibles resemble those of the Cumacea much more closely than 

 those of the Arthi'ostraca. We seem forced, therefore, to the conclusion 

 that the base of the Insectan stem cannot be sought above the base of 

 the whole Malacostracan series, and that such characters as the sessile 

 condition of the compound eyes common to the Insecta and Arthi'o- 

 straca have been independently gained. jSTevertheless, convergence 

 of so striking a natui'e could only be possible in two groups somewhat 

 nearly akin to one another. 



But if it is not possible to believe in the origin of insects from any 

 of the orders of Malacostraca as at present developed, there is some- 

 thing in favour of the view that they branched off from the immediate 

 ancestors of the Malacostraca. In connexion with this point, tlie 

 remarkable Tasmanian " Arthrostracous Schizopod " Anaspidcs, 

 lately described by Thomson ('94) and Caiman ('96), is suggestive. In 

 this animal, the mandible is of the Thysanuran- Cumacean type, but it 

 possesses a palp ; while the first maxilla shows much likeness to the 



