STRUOTUEE AND CLASSIFICATION OF THE ARTHROPODA. 551 



their terga with that of the prosthomeres. But in higher 

 Crustacea the cephalic "tagma" is extended, and more 

 somites are added to the fusion^ and their appendages adapted 

 as jaws of a kind. 



The Hexapoda are not known to us in their earlier or more 

 primitive manifestations ; we only know them as possessed of 

 a definite number of somites arranged in definite numbers in 

 three great tagmata. The head shows two jaw-bearing 

 somites besides the mandibular somite (V, VI, in fig. 6) — 

 thus six in all (as in some Crustacea), including prosthomeres, 

 all ankylosed by their terga to form a cephalic shield. There 

 is, however, good embryological evidence in some Hexapods 

 of the existence of a seventh somite, the supra-lingual, 

 occurring between the somite of the mandibles and the somite 

 of the first maxillas (4), This segment is indicated embryo- 

 logically by its paired coelomic cavities. It is practically an 

 excalated somite, having no existence in the adult. It is pro- 

 bably not a mere coincidence that the Hexapod, with its two 

 rudimentary somites devoid of appendages, is thus found to 

 possess twenty-one somites, including that which carries the 

 anus, and that this is also the number present in the Mala- 

 costracous Crustacea. 



The Segmental Lateral Appendages or Limbs of 

 Arthropoda. — It has taken some time to obtain any general 

 acceptance of the view that the parapodia of the Chfetopoda 

 and the limbs of Arthropoda are genetically identical struc- 

 tures; yet if we compare the parapodium of Tomopteris or of 

 Phyllodoce with one of the foliaceous limbs of Branchipns 

 or Apus the correspondences of the two aie striking. An 

 erroneous view of the fundamental morphology of the crus- 

 tacean limb, and consequently of that of other Arthropoda, 

 came into favour owing to the acceptance of the highly 

 modified limbs of Astacus as typical. Protopodite, endopodite, 

 exopodite, and epipodite weie considered to be the morpho- 

 logical units of the crustacean limb. Lankester (5) has 

 shown (and his views have been accepted by Professors 

 Korschelt and Heider in their treatise on ' Embryology ') 



