AQUATIC RESPIRATION. 271 



called the branchial arteries; the veins, which con- 

 vey it back, being, of course, the branchial veins. 



The larger Crustacea have their branchiae situated 

 on the under side of the body, not only in order to 

 obtain protection from the carapace, which is folded 

 over them, but also for the sake of being attached 

 to the haunches of the feet-jaws, and thoracic feet; 

 and thus participating in the movements of those 

 organs. They may be seen in the Lobster, or in the 

 Crab, by raising the lower edge of the carapace. 

 The form of each branchial lamina is shown at g, 

 in Fig. 354 :* they consist of assemblages of many 

 thousands of minute filaments, proceeding from 

 their respective stems, like the fibres of a feather ; 

 and each group having a triangular, or pyramidal 

 figure. The number of these pyramidal bodies 

 varies in the different genera ; thus the Lobster 

 has twenty-two, disposed in rows on each side of 

 the body ; but in the Crab, there are only seven 

 on each side. To these organs are attached short 

 and flat paddles, which are moved by appropriate 

 muscles, and are kept in incessant motion, pro- 

 ducing strong currents of water, evidently for the 

 purpose of obtaining the full action of that element 

 on every portion of the surface of the branchiae. 



In the greater number of Mollusca, these impor- 

 tant organs, although external with respect to the 

 viscera, are within the shell, and are generally 

 situated near its outer margin. They are composed 

 of parallel filaments, arranged like the teeth of a 

 fine comb ; and an opening exists in the mouth for 

 admitting the water which is to act upon them.t 



* Page 237 of this volume. 



t These filaments have, in most instances, the power of producing 



