The nervous system in Molluscs (Figs. 16.68-16.70) presents numerous diversities. It exhibits gradual coming up of complexities from simple to complex which can be marshalled into one perspective—the nervous co-ordination. Prior to the description of the nervous system in different forms of Molluscs a basic plan of the Molluscan nervous system is to be considered first.
Hypothetical Plan of Molluscan Nervous System:
The central nervous system consists of three pairs of ganglia, the cerebral, pedal and pleural ganglia. These ganglia are connected by connectives and commissures. The connectives are the cerebro-pedal, cerebro-pleural and pleuro-pedal.
The commissures are present between the cerebral, pedals and the pleurals. The peripheral nervous system comprises in a pair of visceral ganglia connected by commissures. Another pair of parietal ganglia are connected with this system.
Comparative Account of Nervous System in different Forms:
From this basic scheme, wide range of modifications has taken place. Concentration of the whole system, attainment of deeper mode of nervous co-ordination, attainment of complexities due to torsion, detorsions and abortion or exaggeration of some parts are the main causative factors.
Protoneurous Condition:
This condition of nervous system is of simplest type and more or less resembles the hypothetical form. This condition shows a gradation of complexities. It may be of undifferentiated type as seen in Chiton where the nervous system is without any definite ganglionic formation (Fig. 16.68G). It may be differentiated type where definite ganglionic formation is observed.
The formation of ganglia shows gradual sequence of modification and evolution. In Solenogastres (Neomeniomorpha) the nervous system is ladder-like and the cerebral ganglion is single (Fig. 16.68D), but in Chaetoderma it becomes double (Fig. 16.68C). In Scaphopoda, the nervous system is symmetrical.
The pleuro-pedal connectives become fused with the cerebro-pedal connective. There is no distinct parietal ganglion. In Unio the parietal ganglia are fused with the visceral ganglion, thus forming the viscero-parietal ganglion (see Fig. 16.35).
Streptoneurous Condition:
This particular condition of the nervous system is observed in Gastropods, particularly in Prosobranchs. Lateral twisting of the nervous system occurs with reference to the torsion of the whole pallial complex.
The original parietal ganglia are renamed according to their new positions. When the parietal ganglion is situated above the level of the oesophagus, it is called supraintestinal ganglion and when it is located below the level of the oesophagus the ganglion is designated as infraintestinal ganglion.
The pleuro-vis-ceral connectives are crossed and assume a pattern like the figure of 8. The whole of the nervous system assumes secondary asymmetry. Such a condition in the nervous system in Molluscs is called the Streptoneury. The streptoneurous condition in different Gastropods exhibits gradation of diversifications and complexities.
Simple Chiastoneury:
This condition is observed in Cyclostoma elegans, Atlanta, Chilina, Planorbarius and in many other Gastropods. In these forms the parietal ganglia are replaced by supraintestinal on the left and the infraintestinal on the right side of the body. The pleuro-visceral connectives are interrupted, the original right and left connectives cross each other.
In Pila (see Fig. 16.18), the chiastoneury is diffused because of the fusion of the infraintestinal ganglion with right pleuro-pedal ganglionic mass. The original infraintestinal nerve still persists as an evidence of migration and fusion of infraintestinal ganglion.
Complicated Chiastoneury:
In most Gastropods the circumenteric nerve ring remains more or less the same excepting the tendencies towards shortening of the nerves between the ganglia. In janthina (Fig. 16.69B) the nerves between the ganglia of the circumenteric nerve ring are quite elongated and the ganglia are quite set apart. The pleural ganglia give off pallial nerves to the mantle.
In many cases like Patella (Fig. 16.69D), Cyclophoridae, a circumpallial nerve joins the two pallial nerves to form a complete nerve ring. The pleuro-visceral loop, in most Gastropods, crosses each other to maintain streptoneurous condition. This pleuro-visceral loop exhibits gredt variation amongst different Gastropods.
A very common tendency is the gradual shortening of the nerves between the pleural and the intestinal ganglia (pleuro-intestinal connectives).
In many cases, the shortening of the pleuro-intestinal nerves is so severe that the intestinal ganglia become fused with the corresponding pleural ganglia of the side. In this wav the original crossing of the pleuro- visceral nerves is obliterated and only the uncrossed portion of the pleuro-visceral loop remains.
In Patella (Fig. 16.69D) the pleuro- visceral loop is greatly reduced and is displaced to the right side. The intestinal ganglia are indistinct. The gills and osphradia receive nerves from the pleuro-visceral loop and the intestinal ganglia. Usually these nerves unite with pallial nerves from the pleural ganglia. Such connective adds more complication to the nervous system.
To discuss briefly minor variations in Gastropodan nervous system the following points can be cited:
1. The visceral ganglion is accompanied by additional one or two ganglia as seen in an olivid.
2. The pedal ganglia are usually the largest ganglia in the circumenteric nerve ring. They are connected by a commissure of variable length. In Haliotis (Fig. 16.68A), Patella (Fig. 16.69D) and Pleurotomaria, the pedal ganglia give origin to two elongated pedal nerve cords posteriorly. These two nerve cords are connected by numerous transverse nerves.
3. In Haliotis, distinct intestinal and pleural ganglia are absent. A pair of branchial ganglia is connected with the pleuro-visceral nerve cords by short nerve on the corresponding side.
4. In Fissurella, the right branchial ganglion is connected to the infraintestinal ganglion and the left one is connected with the supraintestinal ganglion.
Chiastoneury with Zygoneury:
In some Diatocardia, there exists connection between the pallial nerves from the pleural ganglion and the nerve from the intestinal ganglia into the mantle. This type of secondary pleuro- intestinal connection is regarded as Zygoneury. The Zygoneurous condition occurs in Trochus, Triton (Fig. 16.69A), Haliotis (Fig. 16.68A) on the left side. Such connection on the right side also exists in some forms.
Chiastoneury with Dialyneury:
In Fanthina (Fig. 16.69B), Cassidaria, Littorina, Zygoneury is present on both the sides. Such a condition is called the Dialyneury.
Ethyneurous Condition:
Amongst the Gastropods, particularly in many Opisthobranchs and Pulmonates, the nervous system becomes secondarily symmetrical from the primarily asymmetrical stage. In the lower Opisthobranchs and in Pulmonates, the streptoneurous condition persists but in higher forms secondary symmetry is more pronounced.
Attainment of secondary symmetry in the nervous system of these forms leads to a condition called Ethyneurous condition. The ethyneurous condition is the result of either detortion or double torsions. A survey of the nervous system in Opisthobranchs and Pulmonates will give the stages of transformation of streptoneurous to the ethyneurous condition.
In Acteon, an Opisthobranch and in Chilina, a pulmonate, the nervous system exhibits typical streptoneurous conditions by showing usual crossing of the pleurovisceral connectives. Figure 16.69C will give the idea of nervous system in Acteon. Another tendency noticed is the anterior concentration of the different ganglia by shortening the commissures and connectives.
In Gastropteron (Fig. 16.70) the nervous system is a detorted type, where the supraintestinal ganglion has moved to become fused with the right pleural ganglion. The infraintestinal ganglion is similarly fused with the left pleural ganglion.
In Aplysia (Fig. 16.68B) the nervous system is secondarily symmetrical, but the different ganglia on the circumenteric nerve ring are well-separated.
The pleuro-visceral connectives come straight to the posterior side to join the infraintestinal and supraintestinal ganglia about the level of the stomach. In Limnaea (Fig. 16.68F) the nervous system is detorted type which is caused by the anterior migration of intestinal and visceral ganglia to become fused with the pleural ganglia.
Cephalopod Grade:
In Cephalopods the nervous system is complex and highly organized (Fig. 16.68E) that is not found in other invertebrates. In Cephalopods a higher grade of concentration of the central nervous system (except Nautilus where un-fused ganglia are noticed) and the formation of ‘brain’ enclosed by cranial cartilage is observed.
The brain of Octopus contains about 108 nerves. The higher organized brain is correlated with the complex behaviour patterns of locomotion and prey capturing method. It also exhibits speed, strength and agility. The experiments on Octopus vulgaris have shown that the animal cannot distinguish two objects but is capable of retaining some memory and also quite intelligent.
Origin of Chiastoneury:
In the anatomical organisation of Gastropods the pallial complex has changed its position and has become shifted gradually forward along the right mantle furrow (Fig. 16.71 A-D). Each ctenidium becomes shifted along with its parietal ganglion. As long as the pallial complex is not moved far forward to the right the pleuro-visceral connective would not cross but only be shifted to the right as observed in Tectibninchia.
Pallial complex is further shifted forward along the mantle furrow till they come to lie quite anteriorly. The original left ctenidium comes to lie on the right and the original right ctenidium dragged its parietal ganglion over the intestine to the left side as the supraintestinal ganglion.
The original left ctenidium has also drawn its parietal ganglion below the level of the intestine to the right side as the infraintestinal ganglion. The pleuro-visceral connectives in which these ganglia lie now cross and give rise to a condition that is designated as chiastoneury.