138 ANIMALS 



nourish the tissues of the digestive tract itself. It is in a fluid 

 state as blood plasma and is available for assimilation by the 

 other tissues of the body. But a large part of these tissues is 

 too far removed from the digestive tract to be nourished by 

 transfusion. A system of vessels becomes necessary for the 

 conduction of the blood plasma and, in addition, a heart to 

 force the blood along. 



328. Respiration. — The smaller organisms absorb enough 

 oxygen through the general surface of the body to supply the 

 needs of all the tissues. But increase in size also brings some 

 of the tissues too far from the surface to be supplied in this way. 

 Moreover, the development of impervious integumentary 

 tissues prevents the absorption of much oxygen through the 

 general surface. There then becomes necessary a special 

 respiratory organ — gills or lungs. From the organs of 

 respiration the oxygen can reach the tissues through the 

 channels followed by the blood plasma, either dissolved in 

 the blood plasma or carried by special vehicles, the red blood 

 corpuscles. 



329. Excretion. — The waste products of metabolism are 

 voided by the smallest animals through the general body sur- 

 face. But this also becomes impossible in the higher animals, 

 where they are taken up by the blood and are then in part elimi- 

 nated through special excretory organs, the nephridia, kidneys, 

 etc. 



330. Reproduction. — Most of the protozoa reproduce by 

 division of the body, or by budding. These methods also 

 occur largely among the lower metazoa, but highly differentiated 

 tissues lose the power of division and the more complex animals 

 have not the power of reproducing in this way. In them there 

 is a special organ, in which undifferentiated tissue is reserved 

 for reproduction. 



331. Organization of the Body. — We thus see that the de- 

 velopment of the animal, i. e., the nerve-muscle mechanism, 



