182 LECTURE XV. 



dominal limbs, as subdivided lamellae, in the Isopoda : the branchial 

 plates expand into vesicles attached to the thoracic feet in the Lamo- 

 dipoda. In the Stomapoda, the respiratory plates are also external, 

 and are appendages of distinct locomotive organs, and each plate is 

 divided into a series of small filaments or tubes ; so as to resemble a 

 broad feather : their position is abdominal. Similarly formed ex- 

 ternal gills are appended to the thoracic segments in the Thysanopoda. 

 In the lobster and crab tribes, the branchiae are protected by the 

 carapace, and are lodged in lateral recesses formed by the apodemata 

 of the thoracic segments. These branchial recesses are parts of a 

 common cavity, lined by an internal fold of the tegumentary mem- 

 brane, and having two apertures of communication with the sur- 

 rounding medium. The posterior aperture lets in the water, which, 

 traversing the branchial cavity, escapes by the one in front. In the 

 crabs the entry is a cleft in front of the base of the chelse or forceps- 

 claw, and is so placed in reference to the prolonged base of that limb 

 as to be closed or opened at the will of the animal. In the lobster 

 and other Macroura, the entry to the branchial chamber is an extended 

 fissure. 



The dynamical part of the respiratory functions is performed by 

 the lamelliform appendages of the second pair of jaws, which are so 

 situated in regard to the outlets of the branchial chambers as to drive 

 out a certain quantity of the water which has been admitted into the 

 chamber. * The loss of the foul water necessitates the entry of fresh 

 water by the inferent orifices ; the respiratory cavity being neither ca- 

 pable of expansion nor contraction. The mode of breathing in the 

 crypto-branchiate or decapod Crustacea is thus the reverse of that in the 

 Batrachia, in which the dynamical mechanism serves only to draw in 

 the respiratory medium in successive quantities. In the higher verte- 

 brata the thorax is so constructed as to execute acts both of inspiration 

 and expiration. 



The branchice in the decapod Crustacea are in the form of long 

 and slender quadrangular pyramids, and consist of either numerous 

 thin plates, or minute cylinders closely arranged perpendicular to the 

 axis of the pyramid. There are nine branchial pyramids on each side 

 in the crabs {Jig- 90. b), two being rudimental ; but the number is 

 more variable, and usually greater, in the Macroura, amounting to 

 twenty-two in the lobster. 



So far as the gills are attached to the bases of the ambulatory or nata- 

 tory legs, they must be influenced by their movements ; and the more 

 active the progress of the Crustacean, the more briskly will its respi- 



* Milne Edwards, Aiinalcs dcs Sciences Nat. toni. xi. 



