Life of Plants and Animals 217 



contractions, forces the blood out at one end and draws 

 it in at the other. In insects the heart is a long tube 

 lying on the dorsal side of the animal, causing an incom- 

 plete circulation, inasmuch as the blood-vessels lead- 

 ing from the heart are not continuous with those lead- 

 ing to the heart. 



Usually the higher the animal in the scale, the more 

 perfect the circulation. In most mollusks the heart 

 consists of one ventricle and two auricles. In fishes 

 it consists of two chambers, an auricle and a ventricle; 

 in amphibians and reptiles, three chambers, two 

 auricles and one ventricle; while in birds and mam- 

 mals the heart has four chambers. 



The Object of the Circulation seems to be, not only 

 to convey the food material from the alimentary 

 canal to the places where it is needed to build up 

 the wasted parts, but also to distribute the heat in 

 warm-blooded animals, making the temperature uni- 

 form. This heat-production depends on the presence 

 of oxygen, which is also necessary to the liberation of 

 other forms of energy, as the energy of motion. The 

 circulation of the blood serves to distribute oxygen 

 to parts where it is needed; and also to remove waste, 

 such as carbon dioxide, and carry it to the respiratory 

 organs, where it is removed from the body. 



Assimilation. Inasmuch as all forms of energy 

 expended by an animal, either as heat, chemical energy, 

 or motion, are derived from the animaPs tissues, involv- 

 ing their disorganization or dissociation, the loss to 

 the tissues must be made good by assimilation of the 

 food prepared by digestion and conveyed by the cir- 

 culation. Assimilation is the work of the individual 

 cells of the organism. It is thought to consist in 

 building the living protoplasm of the cells out of the 

 organic food. In both plants and animals the proc- 

 ess is probably essentially the same. The power of 

 producing energy resides in the cell; and this varies 



