THE SEA SNAIL. 69 
vary glands, one on each side; the extrinsic muscles 
attached to the pharynx asbessiies to the salivary 
ducts, and the sac of the odontophore, or radula, 
projecting backwards from the lower surface of the 
pharynx. Detach this sac and place it on another 
slide with a drop of water. With the help of the 
dissecting microscope open it longitudinally from 
above and spread it out ; place a cover glass on it, and 
press it gently down with the handle of a needle or 
scalpel. Examine with the inch and then with the 
quarter. The tongue-shaped radula will be seen with 
numerous transverse rows of teeth. The quarter 
objective will show that the rachis teeth differ in 
shape from the lateral teeth. Observe the intrinsic 
muscles. 
4. Nervous system.—Two yellow ganglia (the cerebral 
ganglia), joined by a white commissural cord, and 
behind and outside them two others (parieto- 
splanchnic); these four were above or outside the 
cesophagus. Below, in the centre, lie the two pedal 
ganglia, connected together, and joined to the cerebral 
ganglia; and behind these is a single ganglion 
(azygos ganglion) connected with the parieto-splan- 
chnic ganglia. Trace as far as you can the nerve 
fibres from these ganglia. Cut out the nerve collar, 
and examine—first with the inch and then with the 
quarter. Observe the auditory vesicles on the pedal 
ganglia. Hach contains numerous otoliths. If these 
vesicles are examined quickly after the death of the 
animal, the otoliths will be seen in rapid vibration. 
5. Reproductive system.—Take another animal, remove 
it from the shell, and pin out in salt water with the 
dorsal surface downward. A dark coloured vessel 
