Biology and ecology
Biology
The phylum Mollusca is one of several invertebrate (animals without a spine) groups and comprises a wide array of animals including gastropods (snails and slugs), cephalopods (squids, octopuses), and bivalves (clams, oysters). Of this group, the primary focus of this tool is the terrestrial gastropods. In general, snails are often described as those species that possess a shellShell:
A hard, inflexible, calcareous or chitinous structure that vary in size and may either completely encasing the animal, covering some part of it or be internal.
into which they can retract partially or wholly. Slugs may or may not have shells; for those species that do have shells, it is much reduced and may be internal. Also, for those slugSlug:
A snail that either does not possess a shell or has one that is very reduced (no definite coiling) or internal.
species that have external shells, the shellShell:
A hard, inflexible, calcareous or chitinous structure that vary in size and may either completely encasing the animal, covering some part of it or be internal.
cannot host the body of the animal, and no obvious coiling can be observed.
All terrestrial gastropods have sensory organs referred to as tentaclesTentacles:
Sensory projections on the head end of a mollusc. There are generally two pairs; upper (posterior) and smaller, lower (anterior). The upper pair bears the eyes. In many snails the eyes are located at the tips of this structure; however, in Basommatophoran snail species, the eyes are located at the base of the tentacles.
. There are often two pairs: the larger, upper pair (ocular tentaclesTentacles:
Sensory projections on the head end of a mollusc. There are generally two pairs; upper (posterior) and smaller, lower (anterior). The upper pair bears the eyes. In many snails the eyes are located at the tips of this structure; however, in Basommatophoran snail species, the eyes are located at the base of the tentacles.
) bears the eyes at their tips, and the lower pair (oral tentaclesTentacles:
Sensory projections on the head end of a mollusc. There are generally two pairs; upper (posterior) and smaller, lower (anterior). The upper pair bears the eyes. In many snails the eyes are located at the tips of this structure; however, in Basommatophoran snail species, the eyes are located at the base of the tentacles.
) is used as a sensory organ for detecting odors. Some snail species have only one pair of tentaclesTentacles:
Sensory projections on the head end of a mollusc. There are generally two pairs; upper (posterior) and smaller, lower (anterior). The upper pair bears the eyes. In many snails the eyes are located at the tips of this structure; however, in Basommatophoran snail species, the eyes are located at the base of the tentacles.
(i.e., they lack the ocular tentaclesTentacles:
Sensory projections on the head end of a mollusc. There are generally two pairs; upper (posterior) and smaller, lower (anterior). The upper pair bears the eyes. In many snails the eyes are located at the tips of this structure; however, in Basommatophoran snail species, the eyes are located at the base of the tentacles.
). In these species, the eyes are located at the baseBase:
This is the lower or underside of the shell; opposite of apical.
of the sensory tentaclesTentacles:
Sensory projections on the head end of a mollusc. There are generally two pairs; upper (posterior) and smaller, lower (anterior). The upper pair bears the eyes. In many snails the eyes are located at the tips of this structure; however, in Basommatophoran snail species, the eyes are located at the base of the tentacles.
.
The mouth of the animal is located below the tentaclesTentacles:
Sensory projections on the head end of a mollusc. There are generally two pairs; upper (posterior) and smaller, lower (anterior). The upper pair bears the eyes. In many snails the eyes are located at the tips of this structure; however, in Basommatophoran snail species, the eyes are located at the base of the tentacles.
. It contains a specialized structure known as a radulaRadula:
A rasp-like or ribbon-shaped structure that bears rows of teeth used in feeding.
, which is comprised of a mass of chitinous teeth arranged in rows. The radulaRadula:
A rasp-like or ribbon-shaped structure that bears rows of teeth used in feeding.
is used to scrape pieces of food into the mouth of the animal using a back and forth motion.
The reproductive opening (genital pore) of terrestrial gastropods is generally located anterior-laterallyAnterior-laterally:
This is a directional term meaning towards the front, on the side.
. In snails, the genital pore is located on the headHead:
The area of a mollusc's body that has the tentacles, eyes and mouth.
of the animal, just behind the tentaclesTentacles:
Sensory projections on the head end of a mollusc. There are generally two pairs; upper (posterior) and smaller, lower (anterior). The upper pair bears the eyes. In many snails the eyes are located at the tips of this structure; however, in Basommatophoran snail species, the eyes are located at the base of the tentacles.
. Slugs, however, have their genital pore located between the breathing poreBreathing pore:
This is the breathing hole on the right side of the mantle of molluscs. This allows air to pass through to the mantle for gas exchange. (See also pneumostome).
and the headHead:
The area of a mollusc's body that has the tentacles, eyes and mouth.
, and in some cases this structure may be concealed by the mantleMantle:
A fleshy, membranous covering of the anterior portion of the body of a mollusc. It secretes the materials that form the shell.
. Slugs in the family Veronicellidae are a notable exception to this rule. The genital opening of this group is located ventrally, and there are two openings: one that allows access to the female portion of the genitaliaGenitalia:
The reproductive structures of an animal. May refer to either male or female structure.
and another that allows for the eversion of the male portion of the genitaliaGenitalia:
The reproductive structures of an animal. May refer to either male or female structure.
.
In most terrestrial gastropods, both sex organs occur in the same organism; however, there are a few cases where aphallicAphallic:
The state of lacking a penis.
(does not have a penis) specimens of normally hermaphroditicHermaphroditic:
State of having both male and female reproductive organs. (See also Hermaphrodite)
species (e.g., Deroceras laeve) do exist. However, there are a few species in which separate sexes occur (e.g., Marisa cornuarietis).
The mantleMantle:
A fleshy, membranous covering of the anterior portion of the body of a mollusc. It secretes the materials that form the shell.
is a structure that is located on the dorsal surface of the animal, just behind the headHead:
The area of a mollusc's body that has the tentacles, eyes and mouth.
, and it mainly functions to secrete compounds that are used to construct the shellShell:
A hard, inflexible, calcareous or chitinous structure that vary in size and may either completely encasing the animal, covering some part of it or be internal.
. In snails, the mantleMantle:
A fleshy, membranous covering of the anterior portion of the body of a mollusc. It secretes the materials that form the shell.
is not readily noticeable, as it is often restricted to the shellShell:
A hard, inflexible, calcareous or chitinous structure that vary in size and may either completely encasing the animal, covering some part of it or be internal.
. On the other hand, the mantleMantle:
A fleshy, membranous covering of the anterior portion of the body of a mollusc. It secretes the materials that form the shell.
of slugs is readily visible and generally extends over the back of the animal, covering anywhere from 30% (below, A) to 100% (below, B) of the dorsal surface. The mantleMantle:
A fleshy, membranous covering of the anterior portion of the body of a mollusc. It secretes the materials that form the shell.
may extend over the shellShell:
A hard, inflexible, calcareous or chitinous structure that vary in size and may either completely encasing the animal, covering some part of it or be internal.
of a few species of semi-slugs (e.g., Helicarionidae) when they are active, and can be retracted voluntarily by the animal.
The pneumostomePneumostome:
This is the breathing hole on the right side of the mantle of molluscs. This allows air to pass through to the lung for gas exchange.(See also breathing pore).
or breathing poreBreathing pore:
This is the breathing hole on the right side of the mantle of molluscs. This allows air to pass through to the mantle for gas exchange. (See also pneumostome).
is an opening in the mantleMantle:
A fleshy, membranous covering of the anterior portion of the body of a mollusc. It secretes the materials that form the shell.
of the animal that supports gas exchange, by serving as the entrance to the animal’s lung. The pneumostomePneumostome:
This is the breathing hole on the right side of the mantle of molluscs. This allows air to pass through to the lung for gas exchange.(See also breathing pore).
is located on the right side of the animal (i.e., when the animal is positioned with the tail facing the observer, the pneumostomePneumostome:
This is the breathing hole on the right side of the mantle of molluscs. This allows air to pass through to the lung for gas exchange.(See also breathing pore).
is on the right of the observer).
The ventral portion of the animal bears a muscular structure termed the footFoot:
The muscular organ on the undersurface of the body of a mollusc upon which the animal rests or uses to crawl.
, which is used in locomotion. The skin of the entire animal secretes mucus that aids in the movement of the animal and also serves to reduce dehydration. Many terrestrial gastropods will produce copious amounts of mucus in an attempt to evade potential predators or when irritated.