606 ARTHROPOD A SUB-KINGDOM vn 



and structure of the appendages as follows : EranchiaJa, Miiriopoda, and 

 Insecta. These are in turn divided into several classes, all of which have 

 fossTl 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. 



SUB-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 cephalothorax. The genital ducts 

 open to the exterior near the middle of the body, and true nephridia usually occur. 

 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. 1 



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 Crustaces fossiles sous les rapports 



zoologiques et geologiques. Paris, 1822. . 



Milne- JZdwards, H., Histoire naturelle des Crustaces. 3 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. 

 Gferstaecker, 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-trilobitic 



Genera and Species (Ann. N.Y. Acad. Sci., vol. V.), 1889. 

 Grobben, K., Genealogy and Classification of the Crustacea (Sitzungsber. Akad. Wisseusch. Wieu 



Bd. CL, 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. 



