AND PHYSIOLOGY. 



305 



veloped; the most so in the Cephalopods, which are the 

 highest in organization. In these we find a central organ 

 quite like a brain, enveloped by a membrane analogous to 

 the Dura Mater. This system is shown on Fig. 337. The 

 general arrangement is that of three principal pairs of ganglia 

 with nerves proceeding from them. The first pair is called 

 the Cephalic ganglia, which is the largest, and is located 

 above or on the sides of the esophagus, with a collar of nerves 

 surrounding that tube. This gives off nerves to the organs 

 of vision and taste, and to the muscular apparatus of the mouth. 

 The second pair, called the Pedal ganglia, is located beneath 

 the esophagus, giving off nerves to the foot and the organs of 

 hearing, when this sense is not actually located in the foot. 

 The third pair, the Parieto- Splanchnic ganglia, are usually 

 found in the posterior part of the body giving nerves to the 

 muscular and sensitive walls of the body, the respiratory ap- 

 paratus, the heart and large blood vessels. 



670. Nervous System of Radiates —Need of a Nerv- 

 ous System. — Among the higher Radiates a nervous system 

 of inferior organization can be found. For the most part it 

 consists of a ring of nervous Fia 33 8 - 



matter about the mouth, 

 which sends off branches in 

 different directions. " In 

 Medusae the nervous system 

 consists of a simple cord, of 

 a string of ovate cells, form- 

 ing a ring aroung the lower 

 margin of the animal." — 

 Agassiz. (Fig. 338.) Gang- Ganglia, 

 lia, or reservoirs of force can but seldom be found. 



671. Among the Infusoria no nervous system can be de- 

 tected, and if any exist, its participation in the general course 



669. What is the principal arrangement of the nervous system among molluscs? 670. 

 What is the nervous system of the radiates ? 671. How is the nervous system among 

 Radiates ? 



Nervous System of Star-Fish, g, Live 



