ORGANS OF RESPIRATION. 523 



nerves with their microscopic ganglia, as all the other che- 

 mical functions of animal bodies. In the invertebrated classes, 

 the heart is systemic, and rarely assists in transmitting the 

 blood through the respiratory organs, but in all the verte- 

 brated animals these organs receive the blood directly from a 

 muscular ventricle, which thus contributes its force to aug- 

 ment the respiratory function. Respiration is the chief 

 source of animal heat, it is a means of conveying odorous 

 effluvia to the organs of smell in air-breathing animals, it is 

 the principal stimulus to development in all classes, and its 

 organs, by producing vocal sounds, give expression to the 

 inward feelings, and afford a means of intellectual communi- 

 cation at a distance through the organs of hearing. 



SECOND SECTION. 



Respiratory Organs of the Cyclo-neurose or Radiated 

 Classes. 



As there is no distinct sanguiferous vascular system in 

 poly gastric animalcules, they appear to possess no special 

 organ appropriated to the function of respiration, and the 

 aeration of their fluids is effected through the thin pellicle 

 which covers their exterior, or through the mucous lining of 

 their alimentary cavities. The respiratory currents are pro- 

 duced by minute vibratile cilia, variously disposed over the 

 surface of the body, or around the buccal orifice ; and pro- 

 bably lining the alimentary canal, and the peritoneal cavity 

 where a cyclosis of particles is observed. These vibratile 

 filaments, thus early connected with the function of respi- 

 ration, and here serving also as organs of locomotion and even 

 of prehension, appear to form a part of the respiratory ap- 

 paratus of all the higher aquatic invertebrata, and line the 

 pulmonary organs throughout the vertebrated classes. Those 

 polygastrica which are covered externally with a horny sheath, 

 or with articulated laminae of silica, appear to respire by 

 internal currents through the buccal orifice, as in vaginiform 

 zoophytes. The internal cyclosis of the larger forms, as par- 

 ameecium, and the reticulate cutaneous connecting vessels of 

 the compound forms, as volvox, may also aid in this function. 



The entire gelatinous substance ofporiphera is respiratory, 

 and the incessant currents of water through the pores, canals, 



