^ 286. THE CRUSTACEA. 343^ 



6. With the Isopoda, the five pairs of post-abdominal feet are nearly 

 always coiicerned exclusively in the function of respiration. The two 

 multi-articulate cirri of each of those feet, have been changed into plates, 

 which, pointing backwards, are imbricated and applied against the under 

 surface of the last caudal segment, which is usually very large.*"' The form 

 of these plates is sometimes lanceolate, sometimes discoidal or rhomboidal, 

 and they often differ widely in the different sexes of even the same species. 

 Upon the same foot, the external or anterior plate is usually leathery and 

 bristled on its external border ; while the internal or posterior plate is covered 

 with a very thin envelope, and is usually entirely glabrous. This last, there- 

 fore, should be regarded as the proper branchia, of which the first is only the 

 operculum, serving, also, often as a gyratory organ. The first case is 

 observed with the terrestrial Isopoda, where the branchial opercula are fixed, 

 rhomboidal, slightly concave, and completely cover the branchial lamellae 

 preserving them from desiccation.*'^' 



With most of the aquatic Isopoda, on the other hand, this apparatus is in 

 perpetual motion, and the branchiae are often of the same form and size as the 

 operculate plates. The opercula of the first pair are so large that they extend 

 beyond all the rest.*^' With the Idotheoidae, the operculate apparatus has 

 an entirely peculiar structure. The two feet of the last caudal segment 

 are developed into two valves which move laterally like the two folds of a 

 door, and can open and close the branchial cavity, which is provided with 

 five pairs of double plates.*^"' The branchial apparatus of the Bopyrina 

 differs, in many respects, from that of the other Isopoda. With some 

 species, it is reduced to four or five pairs of simple, superposed plates, with- 

 out any accessory organ ; while with others, there are four to six branchiae 

 which, as more or less deeply fissured cordiform plates, or as long and 



XI. fig. i'^. i^.) of Ampkithoe Jilosa, this animal branchial opercula; but the branchial apparatus, 



has, beside the ten round branchial lamellae, a sixth moreover, is composed of only tliree pairs of plate» 



and rudimentary pair on the two posterior feet.* on each side (Treviranus, Verm. Schrift. I. p. 75, 



T For the respiratory organs of the Isopoda, see Taf. X. XII.), while with Sphaeroma, Cymothoa, 



especially Duvernoy and LerebouUet, Ann. d. and allied genera, there are five pairs on each side 



jc. Nat. XV. 1841, p. 177, PI. VI. {Savigny, loc. cit. PI. XI. XII.). 



S With the terrestrial Isopoda, the branchial With some species of Sphaeroma, Cymodocea, 



apparatus is somewhat abortive, for true branchiae Nesea, and Amphoroidea, the branchial plates of 



are wanting beneath the two anterior pairs of oper- the last two pairs of brancliiae, have numerous 



cula, and those bacli of the three posterior pairs transverse plicae, which connect these Sphaeroma- 



are very small and delicate ; see Treviranus, toda with the Poecilopoda {Duvernoy and Lere- 



Verm. Schrift. I. p. 62, Taf. VI. VIII. IX. (Por- Ooiillet, loc. cit. p. 215, PI. VI. fig. 15-23, and 



cellio) ; Savigny, Descript. de I'figypte, loo. Milne Edwards, Hist. d. Crust. III. p. 223, PI. 



cit. PL XII. fig. 7 (Lygia), and PI. XIII. {Tytos, XXXtl. fig. 9). With Seroiis, tlie branchial struc- 



Porcellio and Armadillidium) ; Brandt, Mediz. ture is quite different, the fourth and fifth pairs 



Zool. II. Taf. XV. fig. 35-37 {Porcellio), and Lere- of feet being clianged into broad branchial plates 



boullet, loc. cit. p. 118, PI. IV. fig. 17, PI. V. fig. {Milne Edwards, Arch, du Mus. d' Hist. Nat. II. 



18-22 {Lygidium). This abortion of the bran- p. 21, PI. II. fig. 1-6). 



chiae is compensated with some Oniscidae by the 10 See Rathke, loc. cit. p. 115, Taf. IV. and 



existence of lung-lilce organs. (See below, § 287.) Milne Edwards, Hist. d. Crust. PI. X. fig. 6, 7 



9 Asellus has two very large, common, anterior {Idothea). 



* [ § 286, note 6.] Leydig (loc. cit. Siebold natatory foot of Branchipus, a peculiar and new 



and Kölliker''s Zeitsch. III. p. 289) does not admit structure. This is a roundish, dark-orange-colored, 



that the red pouches, above-mentioned with Apus, pedunculated body, situated on the under side of the 



are of a respiratory character, at least with Arte- leg near the coxal joint. This body is composed of 



mia and Branchipus, where he has examined their large nucleated cells which contain a yellowish 



histological composition. In this connection it may liquid. The use of this structure is unknown. 



be mentioned that this observer has found on each — Ed. 



