CLASS I CRUSTACEA 691 



and structure of the appendages as follows : Branchiata, Myriapoda and 

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

 fossil representatives. As to the origin of the Phylum, Paleontology 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 stock as many Recent forms. The relatively late appearance of 

 Myriapods, which are the most worm-like of all Articulates, may be accounted 

 for by their terrestrial habitat and the destructibility of their body parts. 



SuBPHYLUM 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 midtiarticulate, the basal joints of one or more pairs serving as organs of 

 manducation. 



The branchiate Arthropods include two classes : Crustacea and Arachnida. 



Class 1. CRUSTACEA.^ 



Arthropods of usually agnatic 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 jnirely 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 ttvo or three branches. 



The segmentation of the body is distinct in all except certain parasitic 

 foi'ms, 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 Arachnids, 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 

 the second and third segments, and is frequently strengthened by deposits of 

 carbonate and phosphate of lime. Although the carapace is usually a single 



^ Literature : Brongniart, A., and Bemarest, A. O, Histoire naturelle des Cnistaces fossiles 

 sous les rapports zoologiques et geologiques. Paris, 1822. — Milne Edwards, H., Histoire naturelle 

 (les 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. — Gerstctecker, 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., 

 1889, vol. V. — Grohben, K., Genealogy and Classification of the Crustacea. Sitzungsber. Akad. 

 Wiss. Wieu, 1892, vol. ci. Translated in Ann. and Mag. Nat. Hist. [6J, vol. xi. — Kingsley, J. S., 

 The Classification of the Arthropoda. Amer. Nat., 1894, vol. xxviii. Reprinted in Tufts College 

 Studies, No. 1, 1894, with bibliography. 



