CHAPTER XXVII. 



COMPARATIVE PHYSIOLOGY. 



THE comparative study of the Physiology of the Inverte- 

 brates has not as yet been carried very far, though there are 

 several careful investigations of particular problems. This 

 chapter is an attempt to gather up some of the most im- 

 portant facts, in order especially to show what is sometimes 

 forgotten, that physiology has much to say upon the general 

 problem of the origin and maintenance of particular charac- 

 ters. A short note on abnormal physiological conditions 

 and their bearing upon evolution has also been added. 



The Physiology of the Nervous System has been very 

 fully investigated in several cases among the Invertebrates, 

 and we will therefore begin our survey with it. 



We may say, in the most general way, that the function 

 of the nervous system is to bring the organism into relation 

 with the external world. The mechanism by which this is 

 effected consists typically of three parts: (i) the peripheral 

 nerve endings, which receive the stimuli; (2) the nerves, or 

 paths by which the stimuli are conveyed to or from (3) 

 the central nerve cells. The peripheral end-organs with 

 which we are most familiar are those of eye, ear, and the 

 other special senses ; but we must not forget that the 

 termination of nerve in muscle the so-called end-plate is 

 equally a peripheral nerve ending. All nerves are in com- 

 munication on the one hand with a peripheral organ, and 

 on the other with central cells. 



It is obvious, from the above definition, that neither 

 Protozoa nor Sponges possess a nervous system. For in a 

 Protozoon the receptive and perceptive mechanism is con- 

 tained in the single cell, any part of the protoplasm will 



