344 THE CRUSTACEA. $ 286. 
sometimes branched tubes, project considerably beyond the lateral borders 
of the posterior segments of the body.“ 
7. The Poecilopoda hold a place between the Isopoda and the Deca- 
poda, their branchiae being, as in the first, inserted on the abdominal feet, 
and, as with many of the second, composed of numerous plates. With 
Limulus, the five posterior abdominal feet, which are inserted on the second 
dorsal segment, and changed, as well as the first pair of abdominal feet, 
into very large plates, have upon their posterior surface numerous semi- 
oval, branchial plates lying upon each other. The first pair of feet appears 
to play, also, at the same time, the part of an operculate apparatus. 
8. The Stomapoda, with which the respiratory apparatus is most highly 
developed, have numerous branchial filaments disposed pectinately on a long 
stalk, and. float freely in the water. 
The Squillina have a similar branchial tuft on the anterior surface of 
the external plate of each of the ten swimming fect, which are only the ten 
post-abdominal feet of the posterior part of the body, transformed.” 
With Thysanopoda, only, these branchial tufts are inserted at the base 
of the anterior abdominal feet. 
9. With the Decapoda, all the branchiae are joined together at the base 
of the anterior abdominal feet and of some of the foot-jaws; but at the 
same time they are contained in a special branchial cavity, which is covered 
by the lateral parts of the cephalothorax. Lach of these two cavities com- 
municates externally by two’ fissures. One of these is situated at the 
under surface of the body between the lower border of the cephalothorax 
and the base of the feet ; through it the water enters the branchial cavity. 
The other is upon both sides of the masticatory organs, and through it the 
water is ejected. In this last, which is sometimes prolonged into a semi- 
canal, are several multi-articulate cirri and lamellae, which belong to 
the second and third pairs of foot-jaws.“® Their continual motion pro- 
duces a regular current of water from the branchial cavity outwards.” 
As to the number of branchiae, there are wide differences in the various 
families of this order. There may be six, seven, fourteen, eighteen or even 
twenty-one in the same respiratory cavity. When numerous, there are 
usually two or three fixed on the four posterior foot-jaws, three or four on 
11 Both sexes of Bopyrus squiliae have five pairs 
of small branchial plates lying over each other like 
scales (Rathké, De Bopyro, &c., p. 7, Tab. 1). 
This is probably true also of the males of Phryrus 
hippolytes (Rathké, Nov. Act. Nat. Cur. XX. p. 
48). The females of this same species and of 
Phryxus paguri have four pairs of cordate, and 
nearly double plates, which stand off laterally a 
little from the posterior part of the body; see 
Rathké, Ibid. p, 46, 59,Tab. I1.; Kroyer, Naturhist. 
Tidskr. III. p. 102, Pl. L. IL., or in Isis, 1841, p. 
693, 707, Taf. IL. Tab. 1, and Taf. IIL. Tab, 2, or 
in Ann. d. Sc. Nat. XVII. 1842, p. 142, Pl. VI. 
With Cepon, the branchial apparatus is highly de- 
veloped in that, beside the five pairs of lanceolate and 
pretty long plates which project from the sides of 
the tail with the males, the five abdominal and the 
last caudal segment, have six pairs of long, narrow 
diverging lamellae with pectinated borders. Du- 
vernoy (Ann. d. Sc. Nat. XV. 1841, p. 120, Pl. 
IV. fig. 1-11), has described these twelve append- 
ages as the principal branchiae of Cepon, while 
to me, they appear to be accessory, and are, 
perhaps, vortex-producing organs — the result of a 
metamorphosis of the anal feet. With Jone, all 
the abdominal segments have a pair of long bran- 
chial tubes pointing backwards, and with the 
females of this same genus, the five anterior pairs 
are branched on one side. In this sex, also, the 
organization of the Amphipoda appears to be re- 
peated, for, from the base of the anterior feet hangs 
along riband-like band (branchia?), See Milne 
Edwards, Hist. d. Crust. IL. p. 279, Pl. XX XIII. 
fig. 14, 15. é 
12 See Van der Hoeven, loc. cit. p. 19, Pl. I. 
fig. 10, PL IL. fig. 1, 11-15; and Duvernoy, Ann. 
d. Sc. Nat. XV. 184%, p. 10, Pl. TLL. 
13 Squilla and Squillerichthus ; see Trevira- 
nus, Beobacht. aus d. Zoot. u. Physiol. p, 22, Taf. 
VI. ‘fig. 36-39; and Milne Edwards, Hist. d. 
Crust. Pl. X. fig. 4, Pl. XXVII. fig. 7. 
14 Milne Edwards, Ibid. Pl. X. fig. 3, Pl. 
XXVI. fig. 6, and Ann. d. Sc. Nat. XIX. 1830, p. 
453, Pl. XIX. 
15 With many Brachyura. 
16 See Suckow, loc. cit. Taf. X. fig. 1, p, q., fig. 
2, p. ¥., fig. 3,d.8.e. (Astacus) ; Milne Edwards, 
Hist. d. Crust. Pl. III. fig. 8-10, i. j. (Mata). 
17 For this mechanism of the respiratory organs 
of the Decapoda, see Milne Edwards, Ann. d. 
Sc. Nat. XI. 1839, p. 126, Pl. ITI. Iv. 
