$286. 



THE CRUSTACEA. 



343 



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

 always concerned 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. 4-. 43.) of Amphithoe filosa, this animal 

 has, beside the ten round branchial lamellae, a si.xth 

 and rudimentary pair on the two posterior feet.* 



? For the respiratory organs of the Isopoda, see 

 especially Duvernoy and Lereboullet, Ann. d. 

 Sc. Nat. XV. 1841, p. 177, PI- VI. 



S With the terrestrial Isopoda, the branchial 

 apparatus is somewhat abortive, for true branchiae 

 are wanting beneath the two anterior pairs of oper- 

 cula, and those back of the three posterior pairs 

 are very small and delicate ; see Treviranus, 

 Verm. Schrift. I. p. 62, Taf. VI. VIII. IX. (Por- 

 ceUio) ; Savis^ny, Descript. de I'figypte, loc. 

 cit. PI. XII. fig. 7 {Lygia), and PI. XIII. {Tylos, 

 Porcellio and Arm adiUidimn) ; Brandt, Mediz. 

 Zool. II. Taf XV. fig. 35-37 {Porcellio), and Lere- 

 boullet, loc. cit. p. 118, PI. IV. fig. 17, PI. V. fig. 

 18-22 {Lygidium). This abortion of the bran- 

 chiae is compensated with some Oniscidae by the 

 existence of lung-like organs. (See below, § 287.) 



y Asellus has two very large, common, anterior 



branchial opercula ; but the branchial apparatus, 

 moreover, is composed of only three pairs of plates 

 on each side {Treviranus, Verm. Schrift. I. p. 75, 

 Taf. X. XII.), while with Sphaeroma, C ymothoa, 

 and allied genera, there are five pairs on each side 

 {Savigny, loc. cit. PI. XI. XII.). 



With some species of Sphaeroma, Cymodocea, 

 Nesea, and Amphorotdea, the branchial plates of 

 the last two pairs of branchiae, have numerous 

 transverse plicae, which connect these Sjihaeroma- 

 toda with the Poecilopoda {Duvernoy and Lere- 

 boullet, loo. cit. p. 215, PI. VI. fig. 15-23, and 

 Milne Edwards, Hist. d. Crust. III. p. 223, PI. 

 XXXII. fig. 9). With Serolis, the branchiiU struc* 

 ture is quite different, the fourtli and fifth pairs 

 of feet being changed into broad branchial plates 

 (Milne Edwards, Arch, du Mus. d' Hist. Nat. II. 

 p. 21, PI. II. fig. 1-6). 



10 Sae Rathke, loc. cit. p. 115, Taf. IV. and 

 Milne Edwards, Hist. d. Crust. PI. X. fig. 6, 7 

 {Idotkea). 



* [ § 286, note 6.] Leydig (loc. cit. Siebold 

 and K6lliker''s Zeitsoh. III. p. 289) does not admit 

 that tlie red pouches, above-mentioned with Apus, 

 are of a respiratory character, at least witli Arte- 

 mia and Branchipus, where he has examined their 

 histological composition. In this connection it may 

 be mentioned that this observer has found on each 



natatory foot of Branchipus, a peculiar and new 

 structure. This is a roundish, dark-orange-colored, 

 pedunculated body, situated on tlie under side of the 

 leg near the coxal joint. This body is composed at 

 large nucleated cells which contain a yellowish 

 liquid. The use of this structure is unknown. 

 — Ed. 



