front of the mouth (now called prosthomeres), I, II, III, coelom of as well as of one or more somites behind it the first, second (opisthomeres).
The adhesion of a greater or less number of somites to the buccal somite posteriorly (opisthomeres) is a matter of importance, but of minor importance, in the theory and history of the Arthropod head.
In Diplopoda two opisthomeres - that is to say, one in addition to the buccal somiteare united by a fusion of their terga with the terga of the prosthomeres.
This enumeration would give six somites in all to the head - three prosthomeres and three opisthomeres.
On the other hand, the facts that the Hexapoda and the Chilopoda have triprosthomerous 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 specialized and elaborate in structure 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 specialized terrestrial lines of descent.
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