MORPHOLOGY. 403 



stimulated, and this stimulation is usually and normally 

 effected (how is not known) by nervous impulse. 



Another physiological peculiarity which needs mention 

 is the power of the cells to take only those substances 

 which they need from the circulating fluids and to build 

 them up into compounds for use in the cell itself, as in the 

 case of muscle- and nerve-cells, for use in other parts of 

 the organism, as in the case of the liver and pancreatic cells 

 or for elimination from the body (kidneys, sweat glands, 



etc.). 



So far we have treated of the animal as an automatic 

 self -regulating machine, but in one respect it differs from 

 all machines of human production. No amount of fuel 

 put under the boiler of a steam-engine will cause this 

 mechanism to increase in size or to give rise to other 

 bits of mechanism like itself. The animal machine grows 

 by the taking in of food, and like the steam-engine, it 

 wears out. It, however, has the power of reproducing 

 the kind, by the formation of small parts (either buds 

 or eggs), which eventually grow into animals like the 

 parent which produced them, and thus the species is 

 perpetuated, the young taking the place of the generation 

 which has worn itself out. 



GENERAL MORPHOLOGY. 



We are now in position to . review some of the facts 

 already discovered in the laboratory or described in the 

 accompanying text, to add to them, and to draw some 

 general conclusions. 



Excepting sponges and some Protozoa, each and every 

 animal can be placed under one of two heads. In the 



