318 THE ANIMAL NERVOUS SYSTEM 



over that of the hydra. In comparison with the independent 

 effector mechanism of sponges and with the receptor-effector 

 mechanism of the coelenterates, the annulates possess the receptor- 

 adjustor-effedor mechanism which is characteristic of all the higher 



metazoa. 



Vertebrata. — Comparisons between the nervous systems of 

 vertebrate animals (Fig. 61, p. 109) and segmented worms show 

 that the relationships are similar, except that the sensory neurones 

 of the vertebrate, comparable with the sensory cells or receptors 

 of the earthworm, have been shifted inward to the dorsal root 

 ganglia, retaining connections with the sldn by long processes. 

 In correlation with this, special groups of cells in the skin have 

 become differentiated as receptors that receive stimuH and trans- 

 mit impulses to these receiving fibers of the afferent neurones. 

 The mechanism of cells that function as adjustors within the 

 central system is similar to that in the earthworm, but there 

 are many more of these cells and hence a greater complexity of 

 interconnections. Long-distance transmission of nerve impulses 

 and the centralization of the system to function as a whole, 

 instead of segment by segment, are conspicuous features of this 

 development. The brain of the vertebrate illustrates this in par- 

 ticular, as does the enormous increase in adjustor cells in the 

 spinal cord. Here, again, muscles constitute the greater mass 

 of the effectors, but gland cells are also effectors. In general, 

 ■ the vertebrate nervous system is characterized by a greater 

 specialization of receptors, a greater centralization in the brain, 

 and a larger number of adjustor neurones. In this manner the 

 receptor-ad justor-effector elements of the sensory-neuro-muscular 

 system of animals, from the simplest to the highest, may be 

 compared. Such comparisons indicate the probable course of 

 the evolution of the nervous system. 



Forms of Behavior in Animals 



Tropisms. — The functions of the cells have been indicated in 

 the foregoing account of the structure of animal nervous systems. 

 Something may be said of the behavior, or the activities of the 

 organism as a whole. We have seen that what are called " trop- 

 isms " have been defined as forced responses of lower organisms 

 to simple fonns of stimulation (cf. p. 181); but the term has 



