XIII] 



NERVOUS SYSTEM 



293 



cutting them out has been made in the Cuttle-fish, and it is then 

 found that the animal loses its 

 power of keeping its balance in 

 the water and tumbles about. 



To return to the central 

 nervous system. In the pond 

 snail, Limnaea, the hinder part 

 of the sub-oesophageal nervous 

 masses consists of no less than 

 five ganglia, strung together on 

 a short loop of nervous fibres, 

 which is called the visceral 

 loop. Of these a pair nearest 

 the head are called the pleural 

 ganglia, the next are called 

 the visceral ganglia, and 

 the one at the end the ab- 

 dominal ganglion (Fig. 134). 

 The front and lower part of the 

 sub-oesophageal nervous mass 

 consists of the pedal ganglia, 

 which send nerves exclusively 

 to the foot. Pleural and 

 visceral ganglia can be re- . 

 cognised in the young snail, 

 but they become indistin- 

 guishably joined in the 

 adult. In other Molluscs, 

 such as the Sea-hare 

 (Aplysia), or the Ear-shell 

 (Haliotis), the visceral loop 

 is long and the ganglia 

 widely separated. In these 

 animals it can be seen that 

 the pleural ganglia send 

 nerves to the sides of the 

 body, and that from the F IG . 134. 

 visceral ganglia nerves 

 come off which go to the 

 base of the gill or gills. 

 At the base of each gill 



FIG. 133. Optical section through the 

 auditory vesicle or ear of Pterotrachea 

 friederici, a transparent pelagic Mol- 

 lusc x about 150. After Glaus. 



1. Auditory nerve. 2. Ciliated cells. 

 3. Auditory cells. 4. Large central 

 auditory cell. 5. Supporting cells. 

 6. Otolith. 



Nervous system of Limnaea. After 

 Lucaze-Duthiers. 



1. Cerebral ganglion. 2. Pedal ganglion. 

 3. Osphradial ganglion. 4. Pleural 



ganglion. 5. Abdominal ganglion. 



6. Visceral ganglion. 



