872 SPECIAL PHYSIOLOGY. 



municating by a trunk with the cloaca, and projecting into the perivisceral 

 cavity, are regarded as respiratory organs ; they are contractile and are lined 

 with cilia. 



Cvelenterata. AH of these are aquatic. They possess no distinct circulatory 

 organs, and accordingly no separate respiratory apparatus ; respiration seems 

 to be accomplished by the interchange of oxygen and carbonic acid, indiffer- 

 ently through any part of the external surface, or specially perhaps at certain 

 ciliated portions of the internal surface of their body-cavities, and within the 

 numerous ciliated tubular prolongations of that cavity, which ramify in the 

 soft disc of some of those animals. 



Protozoa. These universally aquatic animals exhibit the same want of dis- 

 tinct respiratory organs, unless the contractile vesicles be of this character. In 

 the Infusoria, more or less of the surface of the body is ciliated. In the Sponges, 

 the tubular passages through the body form respiratory surfaces, being, in 

 many species, provided at certain points with cilia. Lastly, in the Khizopods, 

 or Foraminifera, and in the still humbler Gregarinida, not even cilia are pres- 

 ent ; but the almost structureless or simple cell-like body of these animals 

 must be stimulated, disintegrated, and purified, by direct absorption of oxy- 

 gen, and exhalation of carbonic acid, so far as is necessary for their simple 

 life. 



The presence of vibratile cilia on the respiratory surfaces of the lower forms 

 of most aquatic animals, is a noticeable anatomical fact ; they often occur in 

 the embryonic, if not in the adult condition. They serve, when present, to 

 change continually the stratum of water in immediate contact with the breath- 

 ing surfaces ; and, according to some, their motion is even to be attributed to 

 the active chemical changes which take place on those surfaces. They are 

 not, however, essential, for they are not always present. Their absence from 

 the branchiae of the Cephalopods, amongst Mollusca, and of the Crustacea, 

 amongst Annulosa, form the most striking exceptions. Amongst the aquatic 

 Yertebrata, they are not found on the gills of fishes, unless upon the branchial 

 apparatus of the singular Amphioxus ; they exist on the temporary gills of 

 the Amphibia, but probably not on the permanent gills of the Perennibranchiate 

 forms. Amongst air-breathing animals, they are wholly absent in the sacs or 

 tracheae of the higher Annulosa. In the lowest air-breathing Vertebrata, viz., 

 the Amphibia, they exist in the lungs as well as in the respiratory passages ; 

 but, in all other instances, they appear to be confined to the air-passages and 

 air-tubes, not extending to the air-cells. It must also be remembered that 

 they are found on other organs besides those concerned in respiration. 



ANIMAL HEAT, LIGHT, AND ELECTRICITY. 



ANIMAL HEAT. 



Inorganic bodies have a constant tendency, by losing or gaining 

 heat, to adapt themselves to the temperature of surrounding media or 

 objects. They may also be artificially cooled, or artificially heated, 

 to all possible degrees. The same is true of dead organized bodies, 

 within the limits of combustion, as of a dead tree, or of a dead human 

 body. Living plants and animals also receive, or give off, heat physi- 

 cally, but they have, besides, a common power of resisting external 

 temperatures ; with plants, this power is very feeble, but with animals 

 it is more marked. In the higher animals especially, there is an in- 

 herent power of maintaining a temperature differing from that of the 

 surrounding media, which are in general cooler, but may be warmer, 



