ARACHNIDA: CRUSTACEA. 531 



Johns Hopkins Univ. Circulars, v. (Oct.) 1885. Moult, sexual characters, Koons, 

 American Naturalist, xvii. 1883. For older lit. see Gerstacker, Bronn's Thierreich, 

 v. pt. i. 



Eurypterina, Woodward, Monograph, Palaeont. Soc. 1866-78. 



Trilobita, Wallcott, Bull. Mus. Harvard, viii. 1 880-81 ; Id. Science, iii. 1884; 

 Salter, Monograph, Palaeont. Soc. 1863-83; Woodward, Monograph, Palaeont. 

 Soc. 1883-84. 



The fossil Branchiate groups and other Arachnids are given (with lit.) in Zittel, 

 Handbuch der Palaeontologie, Abth. i, Palaeozologie, ii. 1881-85. 



Heart in Gamasidae (Acarina\ Glaus; Michael, A. N. H. (5), xvii. 1886. 



Respiratory organs of Arachnida, Macleod, Archives de Biologic, v. 1884. 



Coxal gland of My gale, Pelseneer, P. Z. S. 1885; of other spiders, Bertkau, 

 A. M. A. xxiv. 1884, p. 435 > Id. Z. A. ix. 1886, p. 431 ; of Oribatidae, Michael, 

 British Oribatidae, Ray Soc. i. 1884, p. 177, (cf. Journal Roy. Micr. Soc. (2), iii. 

 1883); of ' Phalangidae and Galeodes, MacLeod, Bull. Ac. Roy. Belg. (3), viii. 1884; 

 ofLimulus, Gulland, Q. J. M. xxv. 1885, and Kingsley, ibid, in ' Notes,' &c. 



CLASS CRUSTACEA. 



Aquatic Arthropoda with cutaneous or branchiate respiration: with 

 two pairs of antennae, a limb -bear ing thorax, either free or united more or 

 less to the head, and as a rule a segmented abdomen which may or may not 

 carry limbs. 



The variety of external forms in this class is very great. The head 

 appears to consist of five fused somites, denoted by the presence of two 

 pairs of antennae, a pair of mandibles, and two pairs of maxillae. The 

 number of somites in the thorax and abdomen of Entomostraca is ex- 

 tremely variable. In the Malacostraca the former has eight, the latter 

 six somites with a telson, except in Nebalia (Leptostraca\ where there are 

 eight. And of the eight thoracic somites, one ( Thoracostracd], three or four 

 (Cumacea), five (Stomatopoda), or all (Decapoda) may be united to the head 

 to form a cephalothorax. The somites of the abdomen may fuse as 

 in some Isopoda ; or the abdomen may be rudimentary, e. g. Cirripedia 

 and Laemodipoda (Amphipoda). In the Ostracoda the body is not seg- 

 mented at any time ; and in some fixed or parasitic forms segmentation 

 may become obscured or lost, though in some instances shown by the 

 presence of limbs. 



Each somite is typically ring-like or cylindriform as in Copepoda, 

 but it may be flattened dorso-ventrally or laterally. In many cases certain 

 somites, or all of them as in most Thoracostraca, develope a lateral pleural 

 process, extending outwards above the articulation of the limbs. A process 

 or fold formed by the edges of the last cephalic somite constitutes the 

 shield of Apus, the mantle (with or without calcareous pieces) of the 

 Cirripedia, and the bivalve shell of Nebalia. United pleural processes of 



M m 2 



