VERTEBRATES 5 



terior, into the former of which the blood is poured by the veins, 

 while from the latter it is discharged through the arteries and distrib- 

 uted to the respiratory organs and throughout the body. In mammals 

 and birds, whose bodies maintain a constant, relatively high tempera- 

 ture, without regard to that of the medium in which they are living or 

 the time of year, each of these portions of the heart is subdivided by a 

 septum, so that the heart in these animals is composed of two pairs of 

 chambers, the anterior pair forming the right and left auricles and the 

 posterior pair the right and left ventricles; the right chamber in each 

 pair contains venous blood which has been brought exhausted of oxygen 

 and charged with carbon dioxide from the various organs of the body 

 and the left chamber contains arterial blood brought purged of carbon 

 dioxide and with oxygen renewed directly from the lungs. It is this 

 complete separation of the venous from the arterial sides of the heart 

 which results in the distribution of pure unmixed oxygenated blood 

 over the body and the consequent maintenance in it of a relatively high 

 and also constant temperature. 



In reptiles and amphibians, on the other hand, the ventricle is not 

 thus divided into two completely separated chambers, but there is 

 a communication of greater or less extent between its venous and 

 arterial sides, and in consequence the freshly oxygenated blood in the 

 left division is more or less diluted by the venous and vitiated blood in 

 the right division before it is pumped through the arteries over the body. 

 Metabolism, consequently, which bears a direct relation to the oxygen 

 content of the blood, cannot be so active as in birds and mammals; the 

 body temperature is not maintained at a constant level but is dependent 

 to a certain extent on the temperature of the surrounding medium and 

 varies with it. 



The heart of fishes contains venous blood only, the auricle and 

 ventricle not being divided into right and left halves (except partially 

 so in the Dipnoi) ; the blood is sent to the gills from the heart, where 

 it is oxygenated, and then distributed directly throughout the body 

 without iirst returning to the heart. 



The process of respiration, by which an animal or plant takes 

 in oxygen from the surrounding medium and gives ofT carbon dioxide, is 

 performed by a radically different body surface in vertebrates than in 

 invertebrates. Being a transfusion process it requires a moist surface, 

 and consequently while it may be performed by an aquatic animal 

 through the outer surface it must be carried on by land animals in the 

 interior of the body. The aquatic invertebrates, with the exception of 

 the chordate forms, respire directly with the integument or with organs 



