Anatomy of finfish and shellfish

Unit 13- Mollusca

13.4.7. Nervous System
The nervous system of Sepia, as of all the cephalopods, shows a high grade of organization, attained only by some insects and arachnids among the other invertebrates.
Brain
The brain consists of four typical molluscan ganglionic masses, all concentrated in the head, round the oesophagus, behind the buccal mass, and protected by a cartilaginous "skull". A pair of cerebral or supra-oesophageal ganglia are fused together into a rounded mass,'lying dorsal to the oesophagus. Laterally, they give off a pair of extremely stout optic nerves which at once expand into large kidney shaped optic ganglia of the eyes. A small olfactory gang/ion lies on the dorsal side of each optic nerve. Anteriorly, a pair of slender cerebro-buccal connectives connects the cerebral ganglia to a pair of superior buccal ganglia, which are situated dorsal to the buccal mass, and connected by circurnoesopha geal connectives to a pair of inferior buccal ganglia lying below the buccal mass.

Fig. 9. Sepia officinalis. Nervous system in dorsal view.

A pair of stout circum-oesophageal connectives connect the certhral ganglia to rest of the brain lying ventral to the oesophagus. The suboesophageat ganglionic mass, lying beneath the
oesophagus, is partly divided into an anterior lobe, the brachial ganglion, and a posterior lobe, the pedal ganglion. From the brachial ganglion run forward ten brachial nerves to the arms; this is one of the reasons to regard these arms as portions of the foot. It is also connected to the superior buccal ganglia by paired brachio-buccal connectives and to the cerebral ganglia by cerebro-brachial connectives. The pedal ganglion supplies the funnel.
A pair of pleuro-visceral ganglia is also united to form a single mass lying in contact with the pedal, behind the oeso¬phagus. They give off two pairs of main nerves, directed posteriorly. The visceral nerves, supplying the various internal organs, form a visceral loop from which springs a pair of branchial nerves, innervating the gills and bearing each a branchial ganglion at the base of the gill. The stout pallial nerves innervate the mantle. To bring about rapid and synchronous muscular movements of the arms, siphon and mantle, there is present a system of giant motor neurons, with its centre lying in a median ventral lobe of the fused visceral ganglia.
Stellate ganglia
The palial nerve on either side runs backwards through the neck to the inner surface of the mantle cavity, were it divides into two branches. The outer branch immediately terminates into a large, roughly triangular, stellate ganglion which can be seen without dissection, in front of the ctenidium when the mantle cavity is opened. Several nerves, arising from its outer border, innervate the mantle. The inner branch is connected to the stellate ganglion by two commissures, and innervates the fin.
Sympathetic system
A pair of sympathetic nerves, arising from the inferior buccal ganglion, runs posteriorly along the oesophagus to join gastric ganglion lying between the stomach and the caecum. The gastric ganglion sends nerves to the liver stomach, caecum and intestine.
Sense Organs
The special sense organs are very well developed and comprise paired eyes, statocysts, ciliated olfactory pits and an unpaired gustatory organ.
Eyes
The paired eyes are large, efficient and bulge from the dorso-lateral sides of the head. They bear striking resemblance to those of a vertebrate in that a cornea, iris, lens and retina are present. The lens projects an inverted image on the retina, as in the vertebrate eye. External muscle attachments enable limited movements of the eye. But the embryological development of the cephalopod eye is entirely different from that of the vertebrate eye, so that homologically they are different, for the vertebrate eye is formed as an out¬growth of the brain, while the cephalopod eye is formed by an ectodermal invagination. The similarity between the two is due to convergent evolution, that is, similarity which is not due to phylogeny.
Each eye lies within an orbit formed by cartilages. The outer wall of the eyeball, or the sclerotic coat, is strengthened by cartilage, and covered by a silvery membrane. It extends in front as the contractile iris presenting a large central opening, the pupil, which can be increased or diminished by muscles. Just internal to the iris lies a large, almost spherical lens, consisting of two piano-convex halves, and held in place by a ciliary body. A choroid is absent. The inner sensitive layer, or retina, is somewhat complicated in structure and is composed of a layer of parallel rods, there being no cones. Close behind the eyeball, the optic nerve swells up into the optic ganglion, from which several bundles of nerve fibres are distri¬buted on the posterior surface of the retina. A small optic gland or white body of unknown function lies near the optic ganglion. A true cornea is also lacking. The transparent horny portion of the skin, covering the exposed surface of the eye, is termed as the false cornea. The skin also forms protecting lids. The cavity of the eye is divided by the lens into a small anterior chamber filled with a water-like aqueous humour, and a large posterior chamber containing a jelly-like vitreous humour. The cephalopod eye can accommodate itself to light changes both by modification in the pupil's size and by the migration of pigment in the retina. It can probably A etect colour.

Fig. 11. Sepia. Eye in section.

Statocyts
A pair of statocysrs, which are organs of equilibrium or balance, lies ventral to the pleuro-visceral ganglion, enclosed in the cranial cartilage. Each statocyst is a small spherical body containing a large statolith and a fluid. The inner surface, lined with a flattened epithelium, is raised into numerous processes, so that the cavity of the statocyst has a very irregular shape.
Olfactory pits
The osphradia of the usual type are lacking. Instead, a small ciliated olfactory pit is situated posterior to each eye. The sensory cells of the pit are inner¬vated from the small olfactory ganglion lying close to the optic ganglion.
Gustatory organ
On the floor of the buccal cavity, just in front of the odontophore, is a small elevation covered with papillae. It is said to be the organ of taste.
Finally, tactile or otherwise sensitive cells are also found on tharms, tentacles and elsewhere.