606 ARTHROPODA SUB-KINGDOM VII 
and structure of the appendages as follows :—Branchiata, Myriopoda, and 
Insecta. These are in turn divided into several classes, all of which have 
fossil representatives. As to the origin of the phylum, Palaeontology affords 
no certain evidence. The entire organisation of Arthropods indicates a close 
relationship with Vermes, and especially with the group of Annelid Worms ; 
nevertheless, the differentiation of the Arthropod type must have antedated 
the Cambrian, since several orders of Crustacea are encountered in the oldest 
fossiliferous rocks which are almost as widely divergent from the supposed 
ancestral form as many recent forms. The relatively late appearance of 
Myriopods, which are the most worm-like of all Articulates, may be accounted 
for by their terrestrial habitat and destructibility of their body parts. 
Susp-PHyLuM A. BRANCHIATA. 
Arthropods breathing by means of gills (or lungs or tracheae modified from gills) 
developed always in connection with the appendages. Head and thorax rarely distinct, 
but usually more or less completely united in a cephalothoraz. The genital ducts 
open to the exterior near the middle of the body, and true nephridia usually oceur. 
Malpighian tubes, when present, are derived from the mesenteron. Anterior append- 
ages all multiarticulate, the basal joints of one or more pairs serving as organs of 
manducation. 
The branchiate Arthropods include two classes: Crustacea and Acerata. 
Class 1. CRUSTACEA! 
Arthropods of usually aquatic habitat, and breathing by gills (exceptionally 
through the general body surface) ; with one or two pairs of appendages (antennae) in 
front of the mouth, the first of which is purely sensory, and several pairs of post-oral 
appendages, some of which are modified into organs of mastication. Appendages with 
typically a basal joint (protopodite) giving rise to two or three branches. 
The segmentation of the body is distinct in all except certain parasitic 
forms, where it is lost in the adult stage through degeneration. Usually the 
demarcation between head and thorax is obscure, and the anterior region of 
the body consists of a cephalothorax, the number of whose segments varies 
within wide limits; this being in sharp contrast to the Acerata, where the 
segments are constantly six in number. The cephalothorax is frequently 
covered by a chitinous shell or carapace, developed from the dorsal portion of 
1 Literature : 
Brongniart, A., and Demarest, A.-G., Histoire naturelle des Crustacés fossiles sous les rapports 
zoologiques et géologiques. Paris, 1822. 
Milne-Edwards, H., Histoire naturelle des Crustacés. 38 vols. Paris, 1834-40. 
Woodward, H., and Salter, J. W., Catalogue and Chart of Fossil Crustacea. London, 1865. 
Woodward, H., A Catalogue of British Fossil Crustacea. London, 1877. 
Gerstaecker, A., Crustacea, in vol. V. of Bronn’s Classen und Ordnungen des Thierreichs. Part 1 
(Cirripedia, Copepoda, Branchiopoda, Poecilopoda, Trilobita), Leipsic, 1866-79; Part 2 
(Isopoda to Decapoda), 1881-94. 
Vogdes, A. W., A Catalogue of North American Palaeozoic Crustacea confined to the non-trilobitie 
Genera and Species (Aun. N.Y. Acad. Sci., vol. V.), 1889. 
Grobben, K., Genealogy and Classification of the Crustacea (Sitzungsber. Akad. Wissensch. Wien 
Bd. CI., Heft 2), 1892. Translated in Ann. and Mag. Nat. Hist. [6], vol. XI. pp. 440-473. 
Kingsley, J. S., The Classification of the Arthropoda (Amer. Nat., vol. XXVIII. pp. 118, 220) 
1894. Reprinted in Tufts College Studies, No. 1, 1894, with copious bibliography. 
