STRUCTURE AND CLASSFFICATION OF THE ARTHROPODA. 573 



Renal exci*etory c^ca (Malpighian tubes) are developed 

 from tlie proctodajum (not from meseuteron, as in scorpion 

 and Amphipoda). 



Concluding- Remarks on the Relationships to one 

 another of the Classes of the Arthropoda. — Our 

 general conclusion from a survey of the Arthropoda amounts 

 to this, that whilst Peripatus, the Diplopoda, and the Ai'ach- 

 nida represent terrestrial offshoots from successive lower 

 Qfrades of primitive aquatic Arthropoda which are extinct, 

 the Crustacea alone present a fairly full series of representa- 

 tives leading- upwards from unspecialised forms. The latter 

 were not very far removed from the aquatic ancestors 

 (Trilobites) of the Arachnirla, but differed essentially from 

 them by the hisrher specialisation of the head. We can 

 gather no indication of the forefathers of the Hexapoda or of 

 the Chilopoda less specialised than they are, whilst possessing 

 the essential characteristics of these classes. Neither embryo- 

 logy nor palaeontology assists ns in this direction. On the 

 other hand, the facts that the Hexapoda and the Chilopoda 

 have triprosthomerons heads, that the Hexapoda have the 

 same total number of somites as the nomomeristic Crustacea, 

 and the same number of opisthomeres in the head as the 

 more terrestrial Crustacea, together with the same adaptation 

 of the form of important appendages in corresponding- 

 somites, and that the compound eyes of both Crustacea and 

 Hexapoda are extremely specialised and elaborate in struc- 

 ture and identical in that structure, all lead to the suggestion 

 that the Hexapoda, and with them, at no distant point, the 

 Chilopoda, have branched off from the Crustacean main stem 

 as specialised terrestrial lines of descent. And it seems 

 probable that in the case of the Hexapoda, at any rate, the 

 point of departure was subsequent to the attainment of the 

 nomomeristic character presented by the higher grade of 

 Crustacea. It is, on the whole, desirable to recognise such 

 affinities in our schemes of classification. We may tabulate 

 the facts as to head-structure in Chfetopoda and Arthropoda 

 as follows : 



