298 ANIMAL BIOLOGY. [Part II. 



(pi. g.). Commissures pass on either side of the oeso- 

 phagus to both these ganglia. The three united ganglionic 

 masses are called the nerve-collar, which is invested by a loose 

 sheath. In the dissection from which the diagram was taken, 

 the anterior (a. t.) and posterior tentacles (p. t.) were both in 

 the retracted position. The optic nerve (op. n.) and the nerve to 

 the anterior tentacle have therefore a somewhat backward direc- 

 tion. The optic is distinct, and arises from a knot on the 

 cerebral ganglion. The nerve to the anterior tentacle is a 

 branch of the labial nerve running forward to the lip. A second 

 nerve has a somewhat similar course, and a small nerve passes 

 over the buccal mass to the integument in the neighbourhood 

 of the horny jaw. In addition to these an auditory nerve runs 

 between the two commissures to the auditory organ (au.\ which 

 is supposed to be seen in the diagram through the substance of 

 the parieto-splanchnic ganglion ; and a buccal nerve runs forward 

 to a buccal ganglion which lies close to the termination of the 

 salivary duct. The two buccal ganglia are united by a delicate 

 commissure. 



From the pedal ganglia a number of pedal nerves (pd. n.) run 

 out to innervate the foot. From the parieto-splanchnic ganglia 

 pallial nerves (pi. n.) proceed on either side, and sooner or later 

 branch before being distributed to the neighbourhood of the 

 collar. Closely associated with the aorta (ao.) is a visceral nerve 

 (v. ??.), which innervates the organs of generation, a twig passing 

 to the heart. Another nerve, taking its origin near the aorta, 

 passes to the floor of the pulmonary chamber. 



Special Senses. The eyes are borne by the posterior tentacles. 

 One of these organs is seen in diagrammatic section in Fig. 86, C. 

 Beneath the external corneal layer (co.\ is the crystalline lens 

 (/.). Beneath this is the retina (r.), the rods of which are sur- 

 rounded by pigmented material. The retina is continuous with 

 the optic nerve (op. n.) Notwithstanding the possession of so 

 well-developed an eye, the snail does not seem to be aware of 

 the presence of a small object, such as the head of a pin, until 

 it is within from Jth to ^th of an inch from the end of the 



