Instead of a single brain, C. hortensis, like other gastropods, has a series of ganglia (nerve knots) on a nerve cord that runs in a ring around the oesophagus. Each ganglion innervates a usually proximate, specific part of the body. The cerebral ganglia on the nerve ring connect to the ocular tentacles, buccal mass and other parts of the head.
The ocular tentacles, and other organs such as anterior tentacles, dart sac and penis, are withdrawn into the body by the pull of retractor muscles that invert the organs like a sock pulled outside in. They are everted out of the body by muscles increasing the hydrostatic pressure of the haemolymph (invertebrate blood) in them. This is possible because the haemolymph is not confined to veins and arteries, but occupies the haemocoel (body cavity).
1: nerve ring.
2: cerebral ganglion.
3: nerve cord containing optic nerve from cerebral ganglion to ocular tentacle.
4: retractor muscle of right lower tentacle.
5: inverted/withdrawn, left, ocular tentacle in natural position.
6: severed retractor muscle of inverted/withdrawn, right, ocular tentacle brought forward.
7: eye.
8: inverted tentacle that is everted for extension; tuberculate structure of tentacle surface is visible within.
9: dark retractor muscle which is pushed by hydrostatic pressure into the tentacle, causing it to evert with the eye positioned at the tip of the tentacle. (See image 4Ch 4 Cepaea hortensis. Juvenile. 11.5mm high, 14mm wide. Pennine foothills, V.C. Derbyshire, England. August 2018. ).
10: oesophagus.
11: buccal mass occupies most of head.
19 Cepaea hortensis. Nerve ring. Pennine foothills, V.C. Derbyshire, England. August 2018.
