332 



CRUSTACEA. 



The branchial chambers are in free communication with the ex- 

 ternal medium by means of two large apertures, through one of 

 which the water enters, while it as constantly flows out through the 

 other. The afferent canal is generally a wide slit that allows the 

 water freely to penetrate to the interior of the branchial cavity; but 

 the passage whereby the respired fluid escapes after passing over the 

 branchiae is provided with a valvular apparatus so disposed as to 

 produce a continual current in the water contained in the chamber, 

 and thus, by insuring its perpetual agitation, effectually provides 

 for its constant renewal. The mechanism is as follows : The 

 aperture by which the water issues is in the neighbourhood of the 

 mouth, and is closed by a broad semi-membranous plate (flabel- 

 lum) derived from the root of the second pair of foot-jaws ; so that 

 every motion of these foot-jaws impresses a corresponding move- 

 ment upon the valve-like flabellum, and in this manner urges on the 

 passage of the water out of the cavity in which the branchiae are 

 lodged. 



But there are other means whereby the action of the limbs is 

 made to assist in the perfection of the respiratory process. Thus, 

 in the lobster, the third pair of foot-jaws, and each pair of ambula- 

 tory legs, except the last, supports a flabelliform plate (Jig. 159, n); 

 the movements of which must likewise keep the fluid respired 

 in a state of agitation, and moreover, by gently squeezing and com- 

 pressing the respiratory tufts, powerfully contribute to the per- 

 fect renovation of the water in contact with the surfaces of the 

 branchi*. 



In the crab genera the 

 arrangement is slightly mo- 

 dified, for here there are 

 three flabella derived ex- 

 clusively from the roots of 

 the foot-jaws (Jig. 156, b, 

 c, d) : of these, two are im- 

 bedded among the bran- 

 chiae ; while the third, as 

 represented in the figure, 

 extends in a crescentic 

 form over the external 

 surface of the whole series 

 of those organs. The 

 end answered in this case is obviously the same as that accom- 





