CRUSTACEA. 321 



In Limulus the two outer thirds of the posterior surface of the 

 basilar joint of the abdominal prolegs are occupied by a great gill 

 formed of a number (130 — 150) of folds of skin, disposed transversely 

 and piled on one another like the leaves of a book : they adhere 

 throughout the length of their base or anterior border, are elsewhere 

 free, are semicircular or triangular, with curved borders, and 

 increase in size from the upper end of the branchia to its base, so as 

 to give this last the form of a pyramid. The free edge of each leaf 

 is rounded, and furnished with a horny band destined to sustain it, 

 and fringed with cilia : elsewhere the gill is membranous. 



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

 carapace, and are lodged in lateral recesses {Jig. 132, K) formed by 

 the apoderaata of the thoracic segments. These branchial recesses 

 are parts of a common cavity, lined by an internal fold of the 

 tegumentary membrane, and having two apertures of communication 

 with the surrounding 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 

 chela3 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 efferent passage from the 

 branchial cavity passes over the lateral portions of the palate or 

 prelabial area. In the Leucosia group it passes over the median 

 portion of the palate. 



The dynamical part of the respiratory functions is performed by 

 the lamelliform appendages of the second pair of jaws, w^hich 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 

 capable of expansion nor contraction. The mode of breathino- 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 vertebrata the thorax is so constructed as to execute 

 acts both of inspiration and expiration. 



The branchiae in the decapod Crustacea {Jig. 132, g, 71) are in the 

 form of long and slender quadrangular pyramids, and consist of either 

 numerous thin plates, or minute cylinders closely arranged perpen- 

 dicular to the axis of the pyramid. There are nine branchial 



* Milne Edwards, Annales des Sciences Nat. torn. xi. 



y 



