190 THE INDIVIDUAL ORGANISM 



cavity of the segment ahead, and there opens directly into the body 

 cavity or coelom. The other end of the coiled tube opens to the exterior 

 through the body wall. The nephrostome sweeps in liquid and small 

 particles by the action of its cilia, and cilia within the tube assist in 

 propelling liquids to the exterior. Portions of the nephridium are com- 

 posed of cells that take up water and certain dissolved materials from the 

 fluid in the tube and return them to the blood, thus concentrating the 

 wastes and conserving useful substances. The nephridia are richly sup- 

 plied with blood vessels. 



Kidneys. The excretory organs of the vertebrates are the kidneys, 

 which operate in the manner already described for man. 



Coordination and Irritability 



We have seen that the various structures employed to accomplish 

 digestion, respiration, and excretion show a wide range in complexity 

 and efficiency. In even the simplest animals it is essential that all these 

 functions (and others) be coordinated to supply the needs of a complete 

 organism. This need for coordination involves the animal's response to 

 the environment; for the rates of digestion, respiration, etc., are largely 

 determined by stimuli that originate in the environment and (because 

 of the irritability of protoplasm) produce appropriate responses on the 

 part of the animal. In the lower animals, the problem of coordination, 

 although very complex, is relatively simple; but as digestion and other 

 processes come to involve more and more complicated structures, the 

 magnitude of the problem of coordination increases proportionately. In 

 general, the greater the degree of coordination the more varied the en- 

 vironment in which the animal can maintain itself. 



Responsiveness in the Protozoa. The single-celled animals show a 

 surprising degree of adjustment to external factors and have at times a 

 rather complicated behavior. Temperature exerts a general effect on 

 protoplasmic activities, controlling rate of reproduction, locomotion, 

 and physiological processes. Chemical stimuli are important, though 

 there are no special receptors. Mechanical stimuli affect the general 

 irritability of the protoplasm or are received by cilia and flagella (loco- 

 motor organelles found in certain groups of the Protozoa). Protoplasm 

 is often directly responsive to light. The stimuli received by the cell 

 may be conducted from the stimulated point to other parts of the cell 

 without the intervention of any specialized structure. However, in some 

 of the more complex protozoa (Ciliata) a "neuromotor" apparatus is 

 present, which consists of protoplasmic fibrils leading from ciliated regions 

 to zones of specially contractile protoplasm. Characteristic movements 

 are produced in response to all the stimuli mentioned above. 



