56 



NEEVOTJS SYSTEM AND GENERAL SENSATION. 



as in the river snail (Paludina vivipara), fig. 35, consists of 

 two oval lobes, u, u, united by a nervous commissure. From 



the cerebral masses 

 nerves proceed to the 

 eyes, tentacules, and 

 mouth; another gan- 

 glionic centre, the pe- 

 dal, occupies the body, 

 from which fibrils pass 

 to the muscular foot, 

 whilst other ganglia 

 supply the respiratory 

 and digestive organs. 

 In the CEPHALO- 

 PODA, as the cuttle- 

 fish, the brain is 

 still more developed. 



Fig. 35. The anatomy of Paludina vivi- Large optic nerves are 

 para (river snail), a, the foot; 6, the oper- 

 culum, fixed to the posterior part of the 

 foot ; d, the respiratory tube, prolonged under 

 the right tentacule; g, the branchiae ; /, the 

 canal of the mucous organ ; n, the heart and 

 auricle ; p, the pharynx ; q', the second cur- 

 vature of the esophagus ; r', the stomach ; s, 

 first turn of the intestine ; s', the second turn ; 

 s", point where the intestine enters the bran- 

 chial cavity ; v, v, salivary glands ; u, u, supra- 

 esophageal ganglions, which represent the 

 brain ; x, principal nerve to the muscular en- 

 velope. 



the form of a cartilaginous plate, extended over the brain. 

 The ganglia placed beneath the esophagus are very large, 

 and give origin to many branches. Ganglia are moreover 

 scattered among the nutritive organs, which are regarded as 

 belonging to the sympathetic system. 



117. In the radiata, the nervous system is reduced to a 

 single ring, encircling the mouth. It differs essentially from 

 that of the mollusca, by its star-like form and horizontal posi- 

 tion. In the anatomy of Asterias aurantiaca (common sea- 

 star), fig. 36, the typical form of the nervous system of the 

 radiata is shown. We observe the mouth surrounded by a 

 nervous ring ; at the centre of each ray of the body is a 



distributed to thehigh- 

 ly organised eyes, and 

 auditory nerves to the 

 rudimentary ears, and 

 branches are sent to 

 each of the tentacula, 

 eight or ten in num- 

 ber, that surround the 

 head. We find, like- 

 wise, in this class, a 

 rudimentary skull, in 



