PHYSIOLOGY OF THE INVERTEBRATA. 295 



borne in mind that the different nerve-fibres merely act as 

 conductors, the effects depend upon the arrangements or 

 apparatuses at the end of the fibres. 



It is the totality of . the properties which nerve-cells and 

 nerve-fibres are capable of giving rise to, which constitutes 

 innervation. 



When nerve-centres or ganglia are excited the activity or 

 energy produced is not the same in each case. Some produce 

 the sensations of light, sound, pain, &c. ; others are the cause 

 of secretion or locomotion; others are associated with 

 psychical states ; while a fourth exerts an influence over 

 other nerve-centres. These nerve-centres may be classified 

 as follows : (a) " Receptive cent7'es, to which influences arrive 

 which may excite sensations or some kind of activity not 

 associated with consciousness. (6) Discharging centres, 

 whence emanate influences which, according to structures at 

 the other ends of the nerves connected with them, may 

 cause movements (muscles), secretion (glands) , or contractions 

 , of vessels, (c) Psychical centres, connected with sensation, in 

 the sense of conscious perception, feeling, volition, intel- 

 lectual acts, and will, (d) Inhibitory centres, which inhibit, 

 restrain, or even arrest the action of other centres." 



In the majority of cases there are terminal organs at the 

 commencement of sensory, and the terminations of motor 

 nerves. Such organs are seen in the rods and cones of the 

 retina and the terminal plates of muscle ; but in some few cases 

 nerve-fibres may terminate in loops towards the periphery of 

 the body or in the interior of organs. 



We now proceed to describe the nervous systems in the 

 Invertebrata. 



The Peotozoa. 



In none of these animals has any trace of a nervous system 

 been discovered; nevertheless, as nervous elements are nothing 

 more than the products of the diSerentiation of protoplasm, 

 it is logical to assume that certain parts of the protoplasmic 



