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In many gastropods, part of the soft body (the visceral mass) is permanently protected within the hard shell. Although some groups such as the nudibranchs, or sea slugs, are adapted for a completely shell-less life. The main part of the gastropod body is the cephalopedal mass, made up of the head and foot. This is often the only protrusible part in shelled species. Many species possess an operculum on the foot, which can completely close the shell aperture, protecting the gastropod from predation and desiccation.
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Photographs 1a (upper) and 1b (lower). Inner (1a) and outer (1b) surfaces of a Turbo sp. operculum. Actual length 41mm. The horny operculum is borne on the rear part of the foot. It is often used to close the aperture but in some species it is secondarily adapted for digging (e.g. Strombus spp.) or locomotion (e.g. members of the Xenophoridae). The operculum may be spiral, concentric or lamellar in form. Most opercula are flat although some, such as those of Turbo sp., carry a sculptured calcareous overlay. Calcareous opercula are the only type preserved in the fossil record. Click on the thumbnail to view a larger image. |
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The mantle flap is a fleshy hood, which is attached to the rear of the visceral mass. The outer surface of the mantle contains cells that secrete the calcareous shell, and may carry tentacles, which screen the mantle cavity. A fold of the mantle forms the inhalant siphon, which draws in the medium containing oxygen for respiration. In carnivorous species the inhalent siphon may also have a sensory function. Often a shorter exhalent siphon extends out from the mantle enabling expulsion of the exhalent current.
Gastropods possess a clearly defined head, which carries the sensory organs. Most species have a pair of tactile cephalic tentacles and two eyes.
The mantel cavity forms a respiratory chamber and contains the ctenidium, the respiratory organ (gill), which extracts oxygen to aerate the blood. In some groups the mantel cavity also houses two osphradia, small chemosensory organs, each with a nerve running to one of the nerve centres, or ganglia, in the head. A hypobranchial gland, that secretes mucous to trap sediment for expulsion, may also be present. In ciliary feeders the endostyle secretes mucous that traps plankton. In most aquatic gastropods water travels in through the inhalent siphon, food particles are trapped in mucus within the mantel cavity and consolidated. Inedible particles are expelled out of the exhalent siphon along with waste products expelled from the anus.
In most terrestrial gastropods the mantle cavity is closed to the external environment, forming an internal respiratory chamber (the lungs), which can be enlarged and contracted by muscles on its floor. The tissue of the chamber contains blood vessels that extract oxygen from air entering through the pulmonary orifice.
Most gastropods have blood containing the oxygen transport molecule haemocyanin, which gives it a blue colour. The heart lies within the pericardial cavity and pumps blood around the body.
The nervous system of gastropods comprises several ganglia, connected to other ganglia and the rest of the body by a network of nerve cords.
The chitinous radula is tougher than the other soft parts but like them has not been found to be fossilised. The radula carries between one and 750,000 teeth. The radula rasps at food and is most important in herbivorous gastropods, in which it carries the greatest number of teeth.
Information on gastropod reproductive organs can be found on the page describing modern forms.
Shell morphology
A protective shell is found in most gastropods, although it is reduced and internalised in some groups such as the slugs and absent in others such as the nudibranchs. The calcareous shell is coated in horny conchiolin, which protects it from chemical decay. The closed apical end of a shell is formed first. The shell then grows by increments as material is added at the apertural end.
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Photograph 3. Inner shell surface Haliotis tuberculata, Holocence, Guernsey. Actual length of section 30mm. The solid shell is formed mainly of calcium carbonate crystals intermeshed with protein. The inner layer of the shell consists of thin layers of calcium carbonate which form nacre, or mother of pearl. Light is reflected from the upper and lower surfaces of the crystal layers producing iridescence. Click on the thumbnail to view a larger image. |
Author: Jennifer Hoyal Cuthill
Last updated: 22.11.05
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