NERVOUS SYSTEM OF RADIATA AND TUNICATA. 351 
ganglia can have any controlling power over the rest. All the 
rays have at their extremities what seem to he veiy 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 Yertebrated 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 Yertebrated 
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 Ascidia. At a is seen 
the orifice by which the water enters for sup¬ 
plying the stomach with food, and for aerat¬ 
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. 
