On the other hand, the land crabs are at an immense distance from these simple forms. The record of the Crustacean familytree is, in fact, a fairly complete E .Ps one - the lower primitive members s: of the group are still represented c' by living forms in great abundance.
The existence of this third prosthomere corresponding to the third prosthomere of the Crustacea is a strong argument for the derivation of the Hexapoda, and with them the Chilopoda, from some offshoot of the Crustacean stem or class.
The opinion now most generally held is that the primitive Crustacean type is most nearly approached by certain Phyllopods such as Apus.
Similarly, early forms of the crustacean sub-class Merostomata have been traced to the pre-Cambrian of North America.
It is to be noted, however, that the Trilobita, which, according to the classification here adopted, are dealt with under Arachnida, are not very far removed, except in such characters as the absence of a shellfold and of eye-stalks, from the primitive Crustacean here sketched.
PRAWN, the name of an edible large shrimp-like crustacean in Great Britain usually applied to Leander serratus (see Shrimp).
The reduction of the organism to seven leg-bearing somites, of which the first pair, as in so many Eu-arachnida, are chelate, is a form of degeneration connected with a peculiar quasi-parasitic habit resembling that of the crustacean Laemodipoda.
Protopodite, endopodite, exopodite, and epipodite were considered to be the morphological units of the crustacean limb.
Somite corresponding to the eyes (the morphological nature of which is discussed below), the smallest number of head-somites so united in any Crustacean is five.
The equivalent terms applied by Sars are Anostraca, Notostraca, Conchostraca, involving a termination already appropriated to higher divisions of the Crustacean class, for which it ought to be reserved.