ORGANS OF RESPIRATION. 545 



smaller lamellae, present an extensive surface for the branchial 

 vessels ; and the currents are directed over them, as in fishes, 

 by the rhythmic contraction and expansion of the surrounding 

 parts. The loose valvular margins, extended from the broad 

 base of the syphon, allow of the free ingress, and check the 

 egress of the respiratory currents, by the sides of the palleal 

 opening; and the valvular fold, directed forwards, in the 

 canal of the syphon, prevents the entrance of the water by 

 that passage during inspiration : so that there are already 

 two lateral and one median opening employed in the respira- 

 tion of cephalopoda, as of fishes, although the currents here 

 take an opposite direction ; and the bilateral symmetry of 

 the respiratory organs, perfectly established throughout this 

 class, accords with the numerous other approximations of 

 the cephalopods to fishes, and to the vertebrated sub- 

 kingdom. 



FIFTH SECTION. 



Respiratory Organs of the Spini-Cerebrated or Vertebrated 



Classes. 



The higher vertebrated classes, like the entomoid classes 

 of articulata, are mostly composed of air-breathing animals, 

 the high development and activity of all their organs re- 

 quiring this more effective mode of respiration; but the 

 lowest vertebrata, like the lowest invertebrated tribes of 

 every subkingdom, are limited to an aquatic respiration, in 

 accordance with their simpler structure, and the diminished 

 energy of all their functions. The blood is always propelled 

 through the respiratory organs by the contractions of a 

 muscular ventricle, and these organs always communicate di- 

 rectly with the cavity of the mouth, through which the respira- 

 tory currents, whether of air or water, are chiefly conveyed. 

 As in most of the invertebrata, the respiratory organs are 

 here always placed near the heart, but their openings are 

 never approximated to the anus, as they most generally are 

 in the molluscous classes. The branchiae of vertebrata are 

 always double and symmetrical, like those of the cephalopods, 

 the highest of the invertebrated classes ; and the pulmonary 

 organs, which are sometimes single as in fishes, and even un- 



PART VI. N N 



