344 Proceedings of the Royal Irisli Academy. 



taceans. "We need not hesitate, therefore, to consider the latter corre- 

 spondence as accurate, and to compare the second maxilla (labium) 

 of Insects with the foremost thoracic legs of Crustaceans. That this 

 correspondence is correct is sho^m by the fact that the labial segment 

 in Centipedes (whereof the poison-feet are the appendages) is 

 not fused with the head-capsule, while in the more generalised 

 Insects it is only partially fused. Among the Cnistacea, we find 

 that the foremost trunk-segment is added to the head in the lEala- 

 costraca — notably in the Amphipoda and the Isopoda ; while the same 

 thing seems to have happened among some of the more specialised 

 Trilobites — e.g., Ogygia. Behind the head we find, in a typical 

 modem Insect, three thoracic and ten abdominal segments, all of 

 which must have carried appendages primitively. And embryological 

 researches have shown that eleven limb-bearing abdominal segments 

 and a limbless twelfth anal segment must have been present in the 

 ancestors of Insects. Comparing the segmentation of an Isopod with 

 that of an Insect, we find exactly the same number of body-segments 

 in each. And the hindmost limb-bearing segment of the primitive 

 insect, revealed only by embryological research, is paralleled by the 

 extra segment in the abdomen of the Leptostraca. There is no 

 improbability in the assumption that this segment originally carried 

 limbs. There is every reason, then, for tracing back the Insecta and 

 the iTalacostraca to common Arthropod ancestors with twenty-one 

 limb-bearing segments, and for considering that the abnormally 

 numerous segments found in Centipedes, ilillipedes, and certain 

 Tnlobites and Phyllopods have arisen by increase from that originally 

 limited number. (See Table, pp. 354-5.) 



In comparisons between Insects and Crustaceans, allowance must be 

 made for the possibility that the stalked eyes in the former class repre- 

 sent yet another pair of head-appendages. The fact that they may be 

 abnoi-mally developed as jointed limbs is hard to explain on any 

 other view. They may be matched in the Insectan branch of the 

 Arthi'opoda by the rudimentary appendages of the pre-antennal 

 segment recently described by Heymons ('01) in the embryo Scolo- 

 pendra. This segment, as previously mentioned, is compared by 

 Heymons with the hinder part of the protocerebron in Insects, to 

 which the compound eyes belong. "We see in this correspondence 

 suggestion for a close comparison between the Insectan and the 

 Crustacean compound eye. 



If true kinship between Insects and Crustaceans be thus estab- 

 lished, it remains that the nature of such kinship be discussed. Have 



