. Animal physiology. Physiology, Comparative; Physiology, Comparative. 332 NERVOUS SYSTEM OF RADIATA AND MOLLUSCA. 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 act independently of one another. 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 ganglia to those on either
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 332 NERVOUS SYSTEM OF RADIATA AND MOLLUSCA. 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 act independently of one another. 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 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 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 harmony with one another, 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 presenting this character. In the lowest and simplest animals of this class, we meet with 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 ac- companying figure, which represents one of the solitary Tunicata, the Ascidia. At a is seen the orifice by which the water enters, for supplying the stomach with food, and aerating the blood (§. 1
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