:S44 THE CRUSTACEA. <§. 286. 



sometimes branched tubes, project considerably beyond tlie lateral borders 

 of the ](Ootcrior 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 phxtes lying upon each other. The first pair of feet appears 

 to play, also, at the same time, the pai-t 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 

 stalle, 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 feet, which are only the tea 

 post-abdominal feet of the posterior part of the body, transformed.'^'^' 



With Thysanojjoda, 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. Each 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 squillae have five pairs cliial tubes pointing backwards, and with the 



of small branchial plates lying over each other like females of this same genus, tha five anterior pairs 



scales (Kathk^, De JJopyro, &c., p. 7, Tab. I.), are branched on one side. In this sex, also, the 



Tliis is probably true also of the males of P/ir(/a7MS organization of the Amphipoda appears to be re- 



hippolytes {Rathki, Nov. Act. Nat. Cur. XX. p. peated, for, from the base of tlie anterior feet bangs 



4S). The females of this same species and of a long riband-like band (branchia .''). See Milne 



Phryxus paguri have four pairs of cordate, and Edwards, Hist. d. Crust. III. p. 279, PI. XXXIII. 



nearly double plates, which stand off laterally a fig. 14, 15. 



little from the jjosterior part of the body ; see 13 See f^an der Hoeven, loo. cit. p. 19, PI. I. 



RdthUi, Ibid. p. 46, !y.),'V:;xh. II.; Kröyer, Naturhist. fig. 10, PI. II. fig. 1, 11-15 ; and Duvernoy, Ann. 



Tidskr. in. [). 102, I'l. I. IL, or in Isis, 1841, p. d. Sc. Nat. XV. 1841, p. 10,'PI. III. 



6J:j, 7Ü7, Taf. II. Tab. 1, and Taf. III. Tab. 2, or l-'J Squilla and S<jiti//erichtkus ; see Trevira- 



in Ann. d. Sc. Nat. XVII. 1842, p. 142, PI. VI. nus, Ueobacht. aus d. Zoot. u. Physiol, p. 22, Taf. 



"With O/jon, the branchial ai)paratus is highly de- VI. fig. 36-39; and Milne Edwards, Hist. d. 



vflojied in that, lieside tlie five pairs of lanceolate and Crust. PI. X. fig. 4, PI. XXVII. fig. 7. 



pretty long ))lates which project from the sides of H Milne Edwards, Ibid. PI. X. fig. 3, PI. 



tlie tail with tin; males, the five abdominal and the XXVI. fig. 6, and Ann. d. Sc. Nat. XIX. 1830, p. 



last caudal segment, have six pairs of long, narrow 453, PI. Xl.X. 



diverging lamellae with pectinated borders. Du- IJ With many Brachyura. 



vernoy (Ann. d. Sc. Nat. XV. 1841, p. 120, PI. K See Suokow, loc. cit. Taf. X. fig. 1, p. q., fig. 



IV. fig. 1-11), has described these twelve append- 2, p. r., fig. 3, d. s. e. (^Astacus) ; Milne Edwards^ 



itges as the principal branchiae of Cepon, while Hist. d. Crust. PI. 111. fig. 8-10, i. j. (^Maia). 



to me, they appear to be accessory, and are, 17 For this meclianism of the respiratory organs 



pirhaps, vortex-producing organs — the result of a of the Decapoda, see Milne Edwards, Ann. d. 



nutaniirpliosis of tlie anal feet. With June, all Sc. Nat. XI. 183Ü, p. 126, PI. III. IV. 

 the abdominal segments have a pair of long bran- 



