21 8 THE CRUSTACEA 



in length by five in breadth (270 x 118 mm.), is by far the largest 

 known Isopod. 



PALAEONTOLOGY. 



Fossil remains which may be definitely referred to the Isopoda 

 are comparatively rare, and the little that is known of their 

 morphology leaves their systematic position in most cases doubtful 

 and throws no light on the phylogenetic history of the group. No 

 palaeozoic forms can be referred, with any certainty, to this Order, 1 

 but several genera are known from Secondary rocks. The genus 

 Urda, Miinster, from the Jurassic of Solenhofen, if it be an Isopod, 

 presents very peculiar characters, having large mandibles projecting 

 in front of the head as in the male Gnatkia, which it seems to 

 approach also in the number and relative sizes of the body-somites, 

 although differing in the large size of the eyes. Cyclospliaeroma 

 (Jurassic) and Palaega (Chalk) strongly resemble in general shape 

 the recent Sphaeromidae and Aeginae respectively. Several genera 

 of Sphaeromidae and Oniscoidea are described from Tertiary deposits. 

 A deformation of the carapace of the Brachyuran Palaeocorystes from 

 the Greensand has been supposed to indicate the presence of an 

 Epicaridean parasite. 



AFFINITIES AND CLASSIFICATION. 



The close affinity of the Isopoda with the Tanaidacea, and 

 .through them with the more primitive members of the Peracaridan 

 division, is clear; they represent the termination of one of the 

 lines of divergence from the caridoid type. The most primitive 

 characters, on the whole, have been retained by the Asellota, which 

 have small and complete coxopodites on the thoracic legs, six 

 distinct segments in the peduncle of the antennule, and sometimes 

 a vestigial exopodite on the same appendage. The Cirolaninae, 

 however, have retained, in Bathynomus, a vestige of the inner 

 nagellum on the antennule, and have, in some cases, six segments in 

 the antennal peduncle, while in the more completely segmented 

 abdomen, and probably in the structure of the pleopods, they may 

 claim to be more primitive than the Asellota. 



The structure of the Isopoda is so diversified, and the number of 

 forms included in the Order is so large, that their classification is a 

 matter of some difficulty. The system now most generally adopted 

 is that of Prof. G. 0. Sars, which is given below with some slight 

 modifications. The tribes (here regarded as sub-orders) into which 

 he divides the Order are for the most part natural groups, but they 

 are of very unequal value. Hansen has pointed out that the 



1 The Devonian Oxyuropoda, recently described from Ireland by Carpenter and 

 Swain, is, however, regarded, with considerable probability, as an Isopod. 



