480 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



appear to be all similar to one another in function, as they are in the 

 distribution of their branches ; every one of them sending a large trunk 

 along its own ray, and two small filaments to the organs in the central 

 disk. The rays being all so similar in structure, as to be exact repeti- 

 tions of each other, it would appear that none of the ganglia can have 

 any controlling power over the rest. All the rays (in certain species) 

 have at their extremities what seem to be very imperfect eyes ; and so 

 far as these can aid in directing the movements of the animal, it is ob- 

 vious that they will do so towards all sides alike. Hence there is no 

 one part, which corresponds to the head of higher animals ; and the 

 ganglia of the nervous system, like the parts they supply, are but repe- 

 titions of one another, and are capable of acting quite independently. 

 Each would perform its own individual functions if separated from the 

 rest ; but, in the entire animal, their actions are all connected with each 

 other by the circular cord, which passes from every one of the five gan- 

 glia to those on either side of it. We shall find that, in Articulated 

 and Vertebrated animals, there is a similar repetition of corresponding 

 ganglia on the two sides of the median plane of the body ; and that 

 these are connected by transverse bands, analogous in function to the 

 circular cord of the Star-fish. Moreover, we shall see a like repetition 

 of ganglia, almost or precisely similar in function, in passing from one 

 extremity of these animals to the other ; and these ganglia are con- 

 nected by longitudinal cords, whose function is in like manner commis- 

 sural. -From the best judgment we can form of the actions of the Star- 

 fish, by comparing them with the corresponding actions of higher ani- 

 mals, we may fairly regard the greater number of them as simply reflex ; 

 being performed in direct respondence to external stimuli, the impres- 

 sion made by which is propagated to one or more of the ganglia, and 

 excites in them a motor impulse. How far the movements of these 

 animals are indicative of sensation, we have not the power of deter- 

 mining ; but it may be safely affirmed, that they afford no indication of 

 the exercise of reasoning faculties or of voluntary power. 



850. Perhaps the simplest form of a Nervous system is that pre- 

 sented by certain of the lower Mollusca ; for the body not here pos- 

 sessing any repetition of similar parts, the nervous system is destitute 

 of that multiplication of ganglia which we see in the Star-fish ; whilst 

 the limited nature of the animal powers involves a corresponding sim- 

 plicity in the integral parts of their instrument. The animals, to 

 which reference is here made, form the class Tunicata, which is inter- 

 mediate, in many respects, between the ordinary Molluscs and the Zoo- 

 phytes. They consist essentially of an external membranous bag or 

 tunic, within which is a muscular envelope, and again within this a 

 respiratory sac, which may be considered as the dilated pharynx of the 

 animal. At the bottom of this last, is the entrance to the stomach; 

 which, with the other viscera, lies at the lower end of the muscular 

 sac. The external envelopes* have two orifices ; a mouth, to admit 

 water into the pharyngeal sac ; and an anal orifice, for the expulsion 

 of the water which has served for respiration, and of that which has 

 passed through the alimentary canal, together with the fsecal matter, 

 the ova, &c. A current -of water is continually being drawn into the 



