NERVOUS SYSTEM OF BADIATA AND TUNICATA. 351 . 



ganglia can have any controlling power over the rest. All the 

 rays 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 obvious 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 repetitions of one 

 another, and act independently of one another. Each would 

 perform its own individual functions if separated from the 

 rest ; but, in the entire animal, they are brought into mutual 

 relation by the circular cord, which passes from every one of 

 the five ganglia to those on either side of it. In Man, as well 

 as in all the Vertebrated and Articulated animals, and in some 

 of the Mollusca, there is a like repetition of the parts of the 

 Nervous System on the two sides of the central line of the 

 body ; but the organs are only double, instead of being 

 repeated five times. Still the two hemispheres of the brain, 

 and the two halves of the spinal cord, in the Vertebrated 

 animal, and the two halves of the chain of ganglia, in the 

 Articulated animal, are as independent of one another as are 

 the five separate ganglia of the Star-fish ; and they are made 

 to act in mutual harmony by similar uniting bands of nervous 

 fibres, which are termed commissures. 



435. In the nervous system of MOLLUSCA, we do not meet 

 with any such repetition of parts ; the body itself not pre- 

 senting this character. In the lowest and 

 simplest animals of this group, there exists 

 only a single ganglion, which may be 

 regarded as analogous to any one of the 

 ganglia of the Star-fish ; but in the higher, 

 we find the number of ganglia increased, 

 in accordance with the increase of the 

 functions which they have to perform. 

 The simplest form of the nervous system 

 in this class is seen in the accompanying 

 figure (fig. 181), which represents one of the 

 solitary TUNICATA, the A scidia. At a is seen 

 the orifice by which the water enters for sup- Fig. isi. NERVOUS SYS- 

 plying the stomach with food, and for aerat- TEM OF ASCIDIA - 

 ing the blood ( 114); and at b is the orifice by which the current 

 of water passes out again, after it has served these purposes. 



