MECHANISM OF EESPIEATION. 



397 



replaced by broad lamellae laid one above the other ; but in every other 

 respect the arrangement is the same. 



(1014.) The respiratory organs above mentioned are lodged in two 

 extensive cavities, or branchial chambers, placed upon the sides of the 

 body, covered by the broad shield of the cephalothorax (fig. 203), and 

 lined by a membrane which is reflected upon the root of each branchia, 

 so as to become continuous with the delicate layer that invests every 

 filament or vascular lamella that enters into its composition. 



(1015.) The branchial chambers are in free communication with the 

 external 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 ensuring 

 its perpetual agitation, effectually to provide 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 (JlabeUum) derived from the root of the second pair 

 of foot-jaws ; so that every 



motion of these foot-jaws ^S- 202 - 



impresses a corresponding 

 movement upon the valve- 

 like flabellum, and in this 

 manner urges on the pas- 

 sage of the water out of the 

 cavity in which the bran- 

 chiae are lodged. 



(1016.) 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 ambulatory 

 legs, except the last, sup- 

 ports a flabelliform plate 

 (fig. 204, n), the movements of which must likewise keep the fluid 

 respired in a state of agitation, and moreover, by gently squeezing and 

 compressing the respiratory tufts, powerfully contribute to the perfect 

 renovation of the water in contact with the surfaces of the branchiae. 



(1017.) In the Crab genera the arrangement is slightly modified ; for 

 here there are three flabella, derived exclusively from the roots of the foot- 



Respiratory organs of the Crab : b, c, d, foot-jaws, from 

 the bases of which the fiabella are derived; f,g, h, k, I, 

 ambulatory legs. 



