Patellogastropod Nervous System
(typical of the
Caenogastropod Nervous System
(typical of the superfamily, Muricoidea)
(Left) Anatomic drawings of the nervous system in two different gastropod
orders, taken from Lankester, 1906
(modified). In the upper figure, a simpler, diffuse nervous system is shown. This is
characteristic of primitive grazers like the limpet. In the lower figure, a more
centralized nervous system is shown, which is typical of many advanced, carnivorous snails.
Overall, the nervous system, unlike other organ systems, retains the bilateral symmetry thought to be characteristic of ancestral, primitive molluscs.
Both gastropod clades basically show the following: paired cerebral ganglia; paired
pleural ganglia, innervating the lining of the mantle cavity; paired pedal ganglia,
innervating the foot; and a more variable set of visceral ganglia. All have one or more
"commissures;" i.e., nerve trunks connecting the right and left sides of the
Patelloidea species, in the Clade: Patellogastropda, generally, have two widely separated major ganglia, the pleural and
the pedal, which are connected by rather long nerve trunks (e.g., cerebro-pleural
connectives in upper figure). Ganglion separation of this kind is also characteristic
of bivalves, which also have a relatively simple nervous system.
Species in the more evolutionarily advanced Clade: Caenogastropoda, by comparison, show a migration of the major ganglia to the head
region, where they have become densely consolidated and interconnected without long
trunks. Smaller ganglia, like the buccal ganglia, have now become consolidated in the
cerebro-pleural complex. The overall consequence of this consolidation is to enable
measureable processing intelligence, as discussed below.
Sense organs generally
include: 1) a statocyst (red, in the figures) for
positioning sense; 2) eyes (see for example, Strombus, Pecten, Chiton
eyes), or if not differentiated eyes (yellow, in the
figure), at least photosensitive nerve endings connected via the optic nerves; 3)
tactile or touch receptors located at various nerve endings; and 4) a chemosensitive
"taste" organ, the osphradium (blue, in the upper figure). While the osphradium is
not shown in the lower figure, it would be connected via the short nerve trunk extending below
the visceral ganglion.
Environmental perception in gastropods seems to be based primarily on
chemoreception through the taste organ (Kohn, 1961;
Audesirk & Audiserk, 1985).
|Fairly complex learned behaviors can also be demonstrated, particularly in the higher orders
of opisthobranchs and pulmonates. These include modifications of feeding response by
experience, habituation, sensitization, and associative learning (Beesley et al., 1998; Mpitsos & Lukovich, 1985)
For many years, visualization has been considered to amount to little more than a
primitive response to light or dark. But in the higher orders of opsithobranchs and
pulmonates, the molluscs show orientation with respect to grass stalks or orientation with
respect to celestial cues (Hamilton, 1977; Hamilton & Russell, 1982). It seems that
the "brain" has formed an internal representation of important environmental