THE EDRIOI HTHALMIA. 315 



the posterior part of the intestine, and appear to be urinary 

 organs analogous to the Malpighian caeca of insects. 



°The respiratory organs vary greatly in structure. In most 

 Eclriophthalmia they are simple plates or sacs, the delicacy 

 of the integument of which permits of the free exposure of 

 the blood circulating in them to the air. In the amphipod 

 genus Phrosina, however, the branchiae are composed of rudi- 

 mentary lamellae, attached to an expanded stem, and resem- 

 ble not a little the epipoditic branchiae of Astacus. In some 

 Sphceromidce, Duvernoy and Lereboullet found the branchial 

 endopodites transversely folded, so as to approach those of 



the Xiphosura. 



The exopodites of the abdominal members of the Isopoda 

 frequently cover the modified endopodites, forming opercula, 

 and the first pair of abdominal limbs is, in many genera, al- 

 tered in such a manner as to form one such large operculum 

 for the four pairs which succeed it. In the Idoteidce it is, on 

 the other hand, the sixth pair of abdominal limbs which are so 

 modified as to form the curious door-like opercula which cover 



the gills. 



In certain of the terrestrial Isopoda (PoreelHo, Arma- 

 dilUdium), some of the opercular plates of the branchiae, 

 usually the two anterior pairs, contain curiously ramified cav- 

 ities, which open externally, and contain air. The genus 

 Tylos possesses respiratory organs, which present a still more 

 interesting approximation to those of the purely air-breath- 

 ing Artlculata. They are thus described by Milne-Edwards : 



" The abdomen presents inferiorly a deep cavity, very 

 similar to that of the Sphceromce, in which the five anterior 

 pairs of appendages are lodged ; but this cavity, instead of 

 being completely open below, is imperfectly closed, in its pos- 

 terior half, by two series of lamellar prolongations, which 

 arise from the sides of the inferior faces of the third, fourth, 

 and fifth abdominal segments, and pass horizontally inward ; 

 the first pair of these plates is small, those of the third pair 

 are, on the other hand, very wide, and almost meet in the me- 

 dian line. The four anterior pairs of abdominal appendages, 

 lodged in this cavity, each carry a wide and short quadrilat- 

 eral appendage, the surface of which is raised into a transverse 

 series of laro:e longitudinal elevations, and each of these eleva- 

 tions presents inferiorly a linear aperture leading to a respir- 

 atory vesicle, the parietes of which are covered with a multi- 

 tude of little arborescent caeca. These vesicles when extracted 

 from the interior of the limb closely resemble a brush-like 



