Anatomy of finfish and shellfish (2+1)

Unit 13 - Mollusca

13.3.8. Nervous System
Brain
The nervous system is very highly developed. Several pairs of ganglia are concentrated in the head, to form a large brain encircling the oesophagus, behind the buccie mass. The two cerebral ganglia, lying above the oesophagus, are fused together forming a single round mass. Below the oesophagus lie two ganglia, an anterior pedal and a posterior visceral. The sides of the oesophageal ring are formed by the paired pleural ganglia, lying lateral to the oesophagus, and giving rise on each side to a large optic nerve, leading to the optic ganglion in the eye. Fibres from the pedal ganglia extend anteriorly a short distance to a pro pedal or brachial ganglion which gives rise to 4 pairs of brachial nerves to the 8 arms and a pair of tentacular nerves to the two tentacles. The cerebral ganglionic mass and the propedal ganglion are connected anteriorly with a small supra-buccal ganglion.

Fig. 6- Loligo Nervous system in dorsal view

Stellate ganglia
Posteriorly, the visceral ganglion gives off a pair of visceral and a pair of pallial or mantle nerves. The two visceral nerves fuse together immediately after origin but again separate posteriorly. The mantle nerve on each side runs backwards and outwards and sends a branch to a large stellate ganglion, living on the inner dorsal surface of the mantle near the tip of the gill. Several giant nerve fibres radiate into the mantle from the stellate ganglia and are responsible for its rapid contraction when jet propulsion is required. Being several hundred times larger than vertebrate nerve fibres, these have become quite popular with nerve physiologists; because their large size makes it much easier to use them by inserting tiny electrodes for study of the characteristics of the nerve impulse.