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The clade (phylum) Mollusca is normally divided into seven smaller clades (classes) of very unequal importance; the most important class of living molluscs is the Gastropoda comprising more than 80% of all living mollusc species.
Minor classes
Aplacophora
There are approximately 320 described species in Aplacophora.
Aplacophorans are found throughout the oceans over the world.
They have worm like bodies and usually less than 5 cm. long, a reduced foot, and mantle cavity. Until the mantle cavity and in some forms the radula was well recognized they were thought to be Echinoderms.
Polyplacophora (chitons) (also called Amphineura)
• 8 valves
• Mouth anterior, anus and mantle cavity posterior
• Paired muscles, gills, nephridia, hearts
• Have the likely-ancestral life habit of algae scraping on hard substrate
• A few fossil forms with more elongate and narrow valves: possibly crevice dwellers
The shells of chitons are characteristically divided into 8 transverse, overlapping shell plates or valves. The striped marginal area is the girdle, all that re- mains of the ancient calcareous mantle. The foot is greatly expanded, forming a large flattened sole which is used not only for locomotion but also for maintaining firm contact with the rock surface. Adhesion is effected mainly by the foot under normal conditions but, when the animal is disturbed, the girdle (the soft part surrounding the shell) is also clamped down tightly onto the rock surface.
Chitons feed on small particles of algae on the rock surface which are scraped off using the radula. Feeding normally takes place when covered by the tide, and they tend to be most active when immersed at night. They are normally inactive animals and move only to feed, but often show 'homing behaviour', which enables them to return to the same area of rock.
Monoplacophora
•A single conical valve
• Mouth anterior, anus and mantle cavity posterior
• Paired muscles, gills, nephridia, hearts
• As with polyplacophorans, have the ancestral molluscan life habit
The animals possess a large flattened foot surrounded by the pallial groove, in which are situated 5 or 6 pairs of gills. Internally, the animals possess 8 pairs of dorsoventral pedal retractor muscles, a feature that is known to have existed in fossil forms.
The monoplacophorans show a number of features that relate them to the polyplacophorans, but they also show a number of distinct characteristics such as the possession of 6 pairs of nephridia lying in the pallial groove. The coelom consists of the pericardial cavity and the cavity of the two pairs of gonads.Fossils exist for this class from the early Cambrian, suggesting this was the early mollusk type. NOTE the presence of segmentation, indicating the relationship of mollusks to other Lophotrochozoans such as Annelids.
Scaphopoda (tusk shells)
•
A single tall conical valve, open at both ends
• Gills very reduced: respiration over mantle surface
• Mostly sediment-sitting detritivores and foram-eaters
The head is very reduced and lacks eyes, but is surrounded by numerous thread-like tentacles or captacula. The captacula possess an adhesive knob at their tip and are used to gather small particles of food present in the sand and pass them to the mouth. An elephant tusk shell is hollow and open on both ends, one for the burrowing foot of the animal, one for respiration, open to the surrounding water..
Scaphopods are found mainly in the deep sea, but a few species occur in shallow water.Earliest fossils are from the Devonian.
The Class Gastropoda includes the snails and slugs.
One of the most diverse clades of mollusks, with 80% of molluscan species
Univalve, conical, often coiled shells
Cephalization: head contains sense organs such as eyes and tentacles
Torsion found in some forms: during development, body rotates counter-clockwise
Torsion usually starts in the veliger or winged larvae stage.
Gastropods are by far the largest group of molluscs. Their 40,000 species comprise over 80% of living molluscs. Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior. Some graze, some browse, some feed on plankton, some are scavengers or detritivores, some are active carnivores.
Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups. Many snails have an operculum, a horny plate that seals the opening when the snail's body is drawn into the shell.
This url shows the operculum of various snails.
A few conch use their operculum defensively.
Gastropods often produce mucus that is very important to moving, especially on land.
Genetics
One of the first early mutations discovered was a maternal effect mutation that determined whether shells in the snail Limnaea coiled to the left rather than the right.
Due to orientation of cells after early divisions in embryo.
Be able to explain the following genetics.
Gastropods are characterized by "torsion," a process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted. Torsion takes place during the veliger stage, usually very rapidly. During torsion, most of the body behind the head, including the mantle, mantle cavity and visceral mass are twisted counter-clockwise (when viewed from the dorsal surface as in the figure below) through 180 degrees. Veligers are at first bilaterally symmetric, but torsion destroys this pattern and results in an asymmetric adult. Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms.
Many forms are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species involves an individual sending a dart into the tissues of the other.
"The love dart transmits a secretion produced by the mucous glands in the genital apparatus. The secretions contain a mix of hormones. Those hormones influence the spermoviduct of the receiving snail, the peristaltic movement of which now supports the sperm cells' way up the canal, which have to pass the spermoviduct to reach the sperm pouch at it's end, to be stored until fertilization. This way, a larger part of the sperm cells survive, and thus the chance to inherit the spender snail's own genes is improved. By application of a love dart, the fertilization chances of a snail can be doubled (Chase, R.; Blanchard, K. C.; 2006)."
"The love dart is a tool of male manipulation" (National Geographic, 2002). The Canadian scientist Dr. Ronald Chase has found out that snails do avoid the partner's love dart. He describes the mating play of brown garden snails (Cornu aspersum) sometimes to look like a medieval knights' tournament ("jousting"), both snails circling each other to avoid being hit.
Many marine species have veliger larvae or larvae with wings. Trochophore larvae can develop into veliger larvae.
Nudibranchs have soft bodies and most of them lack an external protective shell.
Some nudibranchs have numerous body projections (called "cerata") increasing the overall body surface and enhancing breathing. Some species extract nematocysts from the coelenterates on which they feed and store them in the special sacs at the tips of their ceratas called cnidosacs.
The lion maned nudibranch have has a large expandable oral hood which is fringed with sensory tentacles.
These nudibranchs are on the following list of deadly slugs and snails http://www.bogleech.com/bio-gastropoda.html
Other nudibranchs produce toxins that appear to discourage predators.http://www.seaslugforum.net/general.htm
The nudibranchs are a groups of snails that have undergone detorsion. http://www.seaslugforum.net/find/detort
Some nudibranchs are masters of camouflage See appropriate references in seaslug forum
