OP ANIMALS. 37 



which sends the blood into one single trunk, both aortic and 

 pulmonary; the greater number have two auricles, the batra- 

 chia have only one. Finally the heart is double in birds and 

 the mammalia, they have two auricles and two ventricles in 

 contact, one aortic and the other pulmonary. 



21. In order that the nutritive fluid may be fitted for its 

 function, it must be submitted to the action of the atmosphere 

 in which the animal lives. In those which have no circula- 

 tion the water acts on the surface of the body; such seems to 

 be the case with the infusoria, polypi, and acalepha: the in- 

 testinal worms also have not the least appearance of respiratory 

 organs. In another degree of organization, air or water pene- 

 trates into every part of the body by elastic canals called tra- 

 cheae, and which are lined by a prolongation of the skin. The 

 echinodermata have aquiferous tracheae; in insects there are two 

 longitudinal tracheae extending throughout the body, having 

 at intervals common centres from which arise many branches, 

 and which correspond to stigmata, or external openings for 

 the entrance of air. In animals that have acirculation, part 

 of the vessels carries the blood into an organ in which they 

 are subdivided over an extensive surface of the external or in- 

 ternal skin. This surface is salient and is called branchiae when 

 the ambient element is water, and lungs, and hollow, when that 

 element is air. In order to carry on the branchial or pulmo- 

 nary respiration, there are generally organs for motion, to put 

 the ambient fluid in contact with the organ. In the arachnides, 

 we find the transition of disseminated respiration, which yet 

 exists in the tracheariae, to the local respiration, which occurs 

 in pulmonary sacs. In the Crustacea generally, the respirato- 

 ry organs are projecting branchiae variously configured. The 

 same is the case with most of the annelides. In the mollusca, 

 generally, we find a very great variety in the organs of respi- 

 ration. Some breathe the air itself, and have a pulmonary 

 cavity; these are the gasteropodia with lungs; others have pro- 

 jecting branchiae variously configured; others again have their 

 branchiae in a cavity into which the water is drawn. In fishes, 

 respiration is branchial; but it is pulmonary in the other ver- 

 tebrated animals. 



