ITS RELATION TO NERVOUS SYSTEM OF IN VERTEBR AT A. 561 



tion of similar parts which are to be alike supplied with nervous power, but to 

 the c-xcrcise of a diversity of functions, through the instrumentality of differ- 

 ent structures : thus, in the higher Articulated and Molluscous tribes, we find 

 ganglionic centres specially set apart for the actions of deglutition and respi- 

 ration, as well as for those of locomotion; but their modus operandi is still 

 the same, these actions being all " excito-motor," that is, being performed 

 through the " reflex " agency of their several ganglionic centres, without 

 the necessary intervention of consciousness. These centres are connected 

 with each other coramissu rally, when they are required to act with con- 

 sentaneousness ; and it is frequently to be observed in the most developed 

 forms of each type, that they come into actual coalescence, their functional 

 distinctness being still indicated, however, by the distribution of their nerve- 

 trunks. 



iv. In all but the very lowest Invertebrata, the Nervous System includes, 

 in addition to the foregoing, certain ganglionic centres, situated in the neigh- 

 borhood or the entrance to the digestive cavity, and connected with organs, 

 which, from their more or less close resemblance to our own instruments of 

 special sense, we conclude to be organs of sight, smell, hearing, etc. Now as 

 we know from our own experience, that impressions made upon these organs 

 produce no influence on our actions unless we become conscious of them, and 

 as the Invertebrata possess no distinct gaugliouic centres of a higher char- 

 acter, it seems to be a legitimate inference, that these " sensorial " ganglia 

 are the instruments by which the animals furnished with them are rendered 

 cognizant of such impressions, and through which the sensations thus called 

 into existence serve to prompt and direct their movements. What is 

 commonly designated as the " brain " of Invertebrata (more properly their 

 "cephalic ganglia") cannot be shown to consist of anything else than an 

 assemblage of sensorial centres; and its actions appear to be entirely of a 

 " reflex ''"character, such of the movements of these animals as are not excito- 

 motor, being performed (there is strong reason to believe) in direct respoudeuce 

 to sensations excited by internal or external impressions. Such movements, 

 therefore, may be designated as sensori motor or consensual. Like the pre- 

 ceding, they do not appear to involve the participation either of Emotion, 

 Reason, or Will ; and the proportion which they bear to the actions of the 

 excito-motor kind, seems to correspond pretty closely with the relative de- 

 velopment of the cephalic ganglia and of the rest of the nervous system, as 

 is very obvious when the larva and imago states of Insects are compared. 

 However disjointed the various "excito-motor" centres may be amongst 

 each other, we uniformly find them connected with the " sensory " ganglia 

 by commissural tracts; and this anatomical fact, with many phenomena 

 w'hich observation and .experiment upon their actions have brought to light, 

 makes it apparent, that besides the reflex actions which are performed 

 through their own direct instrumentality, the sensory ganglia have a par- 

 ticipation in those performed through other ganglionic centres. Thus it 

 seems probable that a stimulus transmitted downwards from the sensory 

 ganglia, to one of the ganglia of the trunk of a Centipede, excites the 

 efferent nerves of that ganglion to call into contraction the muscles sup- 

 plied by them, just as the excitor influence arriving at that ganglion 

 through its own afferent nerves would do. 



448. The whole Nervous System of Invertebrated Animals, then, may be 

 regarded as ministering entirely to purely reflex action ; and its highest 

 development, as in the class of Insects, is coincident with the highest mani- 

 festations of the "instinctive" powers, which, when carefully examined, are 

 found to consist entirely in movements of the excito-motor and seusori-motor 

 kinds. When we attentively consider the habits of these animals, we find 



