GASTROPODA. 89 



producing a kind of trembling of the whole mass 

 which occupies the centre of the capsule, is a won- 

 derful spectacle. It has been recently discovered 

 that these motions are due to very small cilia upon 

 the internal surface of the capsule." * 



The nervous system varies in its development in 

 the genera of GASTROPODA. The central portion 

 consists of a group of ganglions, set near together, 

 surrounding, like a ring, the gullet. In many 

 genera, the ganglions which form the upper part 

 of the ring are blended together, and constitute a 

 brain ; while on the opposite (or lower) side of the 

 ring, there is also another large mass of grouped 

 or blended ganglions. From this ring nervous 

 threads pass in various directions to the organs of 

 sense, along the foot, &c., in some cases connect- 

 ing themselves with other ganglions, which form 

 centres of communication to the stomach and other 

 organs. 



There is always in these animals a distinct 

 mouth, placed at the front of the head, furnished 

 with swollen, contractile lips, often prolonged into 

 a cylindrical proboscis. In many species, as in 

 Eolis, Tritonia, &c., there are two horny, sharp- 

 edged plates, which act as Jaws, playing over each 

 other, like the blades of shears. Sometimes there 

 is only a single jaw-plate inserted in the palate. 



There is, beside the jaws, a fleshy band, longer 

 or shorter according to the genus, which performs 

 the function of a tongue. It is sometimes grooved 

 along its surface, and is always armed with horny 

 teeth or spines, which are arranged in regular rows, 

 both longitudinally and transversely. The form, 



* Siebold's Comp. Anat. ; 211. 



