PHYSIOLOGY OF THE INVERTEBRATA. 371 



the theory, that the Vertehrata have been developed from the 

 Arthropoda rather than from the Molhcsca. (See Chapter X.). 

 In this chapter we have seen that many of the lower 

 animals have tolerably well-developed organs of sense, and 

 as such ol-gans are the means of awakening consciousness, it 

 is reasonable to conclude that, on the whole, every nervous 

 system, however little developed, in the Invertebrata as well 

 as the Vertehrata, may be traced to a conscious cellular part, 

 in continuous relation with two nervous systems, the one 

 afferent, through which sensory excitation is conveyed, the 

 other efferent, by which motor incitation is transmitted. The 

 mode of action of such a mechanism is evidently reflex action, 

 and, in fact, there is not a central nervous act, from the 

 Protozoa to the highest Vertehrata, which cannot be traced to 

 reflex nervous acts. First of all, the reflex action is absolutely 

 unconscious ; but in a later phase the nervous cell becomes 

 conscious of vibration of its molecules ; it experiences the 

 sensations of touch, taste, smell, &c., more or less varied ac- 

 cording as the organ or organs are more or less differentiated. 

 At the same time it has impressions of pain, but in the lower 

 animals these impressions are only momentary — there is no 

 memory. Later still, however, this faculty becomes mani- 

 fested ; which is followed by the co-ordination of impressions, 

 sensations, &c., in other words — understanding, intelligence, 

 or reason, comes into play. But behind all this labyrinth of 

 psychical phenomena there are simply reflex acts, transformed 

 sensations and impressions. It has often been stated that 

 animal " intelligence " is merely due to instinct and not to 

 reason; that instinctive actions are not the result of ex- 

 perience or of previously acquired knowledge through the 

 sensesj whilst those of reason, can be readily traced to these 

 sources. Many acts of the Insecta and the Arachnida, for 

 example, such as slave-making, cell-making, web-making, &c., 

 are described as due to instinct ; but there are many actions 

 among these Invertebrates which appear to come under 

 the head of reason. Among these may be mentioned the 



