§ 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 asthe 
operculate plates. The opercula ofthe 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 paivs 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°. 48.) of Amphithoe filosa, this animal 
has, beside the ten round branchial lamellae, a sixth 
and rudimentary pair on the two posterior feet.* 
7 For the respiratory organs of the Isopoda, see 
especially Duvernoy and Lereboullet, Ann. d. 
3c. Nat. XV. 1841, p. 177, Pl. VI. 
8 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 T'reviranus, 
Verm. Schrift. I. p. 62, Taf. VI. VIII. LX. (Por- 
cellio); Savigny, Descript. de VEgypte, loc. 
cit. Pl. XII. fig. 7 (Lygia), and Pl. XIII. (Tylos, 
Porcellio and Armadillidium) ; Brandt, Mediz. 
Zool. Il. Taf. XV. fig. 35-37 (Porcellio), and Lere- 
boullet, lov. cit. p. 118, Pl. IV. fig. 17, Pl. 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.) 
9 Asellus has two very large, common, anterior 
* [ § 286, note 6.] Leydig (loc. cit. Siebold 
and Kélliker’s Zeitsch. III. p. 289) does not admit 
that the red pouches, above-mentioned with Apus, 
are of a respiratory character, at least with drte- 
mia and Branchipus, where he has examined their 
istological positi Tn this tion it may 
be mentioned that this observer has found on each 
branchial opercula ; but the branchial apparatus, 
moreover, is composed of only three pairs of plates 
on each side (T'reviranus, Verm. Schrift. I. p. 75, 
Taf. X. XII.), while with Sphaeroma, Cymothoa, 
and allied genera, there are five pairs on each side 
(Savigny, loc. cit. Pl. XI. XII). 
With some species of Sphaeroma, Cymodocea, 
Nesea, and Amphoroidea, the branchial plates of 
the last two pairs of branchiae, have numerous 
transverse plicae, which connect these Sphaeroma- 
toda with the Poecilopoda (Duvernoy and Lere- 
boullet, loc. cit. p. 215, Pl. VI. fig. 15-28, and 
Milne Edwards, Hist. d. Crust. III. p. 223, Pl. 
XXXIL. fig. 9). With Serolis, the branchial struc- 
ture is quite different, the fourth and fifth pairs 
of feet being changed into broad branchial plates 
(Milne Edwards, Arch. du Mus. @’ Hist. Nat. IL. 
p. 21, Pl. IL. fig. 1-6). 
10 See Rathkeé, loc. cit. p. 115, Taf. IV. and 
Miine Edwards, Hist. d, Crust. Pl. X. fig. 6, 7 
(ldothea). 
natatory foot of Branchipus, a peculiar and new 
structure. This is a roundish, dark-orange-colored, 
pedunculated body, siti d on the under side of the 
leg near the coxal joint. This body is composed of 
large nucleated cells which contain a yellowish 
liquid. The use of this structure is unknown 
—Ep. 
