COORDINATION IN ANIMALS 183 



that the thyroid gland itself is subject to regulating stimuli 

 reaching it through the nervous system, as well as by a hor- 

 mone derived from the PITUITARY BODY which is another 

 endocrine gland situated in conjunction with the lower part 

 of the brain. Glimpses of such interrelationships are being 

 gradually afforded as one hormone after another is discovered. 

 But chemical coordination, indispensable as it is as a means 

 of regulating many of the processes of the organism, espe- 

 cially the slower ones such as growth, is entirely inadequate 

 for the instantaneous correlation of diverse parts of an animal 

 and also for the adjustment of the whole animal to its sur- 

 roundings. The nervous system supplies this need by a com- 

 plicated arrangement of cellular elements in which irritability 

 and conduction are highly developed. (See p. 206.) 



B. COORDINATION BY THE NERVOUS SYSTEM 



In some unicellular organisms certain portions of the 

 protoplasm are especially differentiated for receiving and 

 conducting stimuli, and others for making effective such 

 stimuli by contractions of the whole or parts of the cell. 

 It is in the lower Metazoa, such as Hydra and its allies, 

 however, that we find the establishment of definite NERVE 

 CELLS some of which are specialized for receiving stimuli and 

 others for conducting the excitation to cells specialized 

 for contracting (muscle cells), etc. Thus a simple RECEPTOR- 

 EFFECTOR system arises which may be regarded as the basis 

 for the development of the elaborate NEURO-MUSCULAR 

 MECHANISM of higher forms. Although from the functional 

 point of view it is impossible to differentiate between the re- 

 ceiving and conducting elements and those which make them 

 effective (muscular system) in the economy of the organism, 

 from the standpoint of anatomy the former constitutes a 

 definite entity, the NERVOUS SYSTEM proper. (Figs. 99, 100.) 



