Question

8 shaped nervous system belongs to
A. Pila
B. Unio
C. Anodonta
D. All of the above

Answer 1

Hint: The organism has an asymmetric nervous system due to the torsion of the visceral hump and is one of the largest freshwater mollusks.

Complete answer:
The nerve from the left pleural ganglion connects with the right pleura-visceral connective forming an infra intestinal ganglion i.e. the visceral loop is twisted into a figure of 8. This occurs due to the torsion of the visceral hump. This arrangement is commonly called an 8 shaped nervous system and is characteristic of Pila.

Additional information:
The nervous system of Pila is well-developed having 5 pairs of ganglia (Cerebral ganglia, Pedal ganglia, Visceral ganglia, Pleural ganglia, Supra, and infra intestinal ganglia), the connectives and commissures, and nerves.
The structure is explained as:
- Cerebral ganglia are present on the dorsolateral sides of the buccal mass connected by cerebral commissures.
- A pair pleural and a pair of pedal ganglia are present on the ventrolateral sides of the buccal mass. Cerebro-pleural connectives and Cerebro-pedal connectives connect the cerebral ganglia & pleural ganglia and the cerebral & pedal ganglia respectively. Pedal commissure connects the two-pedal ganglia. All these structures will form a nerve ring that looks like a square.
- Two buccal ganglia are present at the junction of buccal mass and esophagus. They are connected in the cerebral ganglia with connectives.
- Two visceral ganglia are present in the visceral mass. The pleural ganglia and visceral ganglia are connected by right and left pleura-visceral connectives. A supra intestinal nerve will arise from the right pleural ganglion which will travel over the esophagus and is connected to the left pleura-visceral connective forming a supra intestinal ganglion.
- The left pleural ganglion gives a nerve that connects with the right pleura-visceral connective and an infra intestinal ganglion is formed. Thus these nerves will give '8' shaped structure.
So, the correct answer is (a) Pila

Note:
It is a primitive feature because the ganglia and connectives show secondary bilateral symmetry in most of the gastropods. Cerebral nerve ganglia result in several small nerves that reach tentacles, eyes, statocysts, etc. The mantle is innervated by nerves from pleural ganglia and excretory, reproductive, and stomach is innervated by nerves from visceral ganglia.