CSS Forums - Foot Types in Mollusca

Mollusks have a wide range of locomotory patterns. Solenogasters and various smaller gastropods glide upon cilia that beat rapidly against a pathway of mucus secretions. This pattern of movement is supported or replaced in larger mollusks by the propulsive waves that run along the surface of the foot and are controlled by the actions of the dorsoventral musculature. Burrowing occurs as an interaction between musculature and the hydrostatic skeleton it is performed in caudofoveates and several sea slugs by the whole anterior body but is restricted to the foot in scaphopods, bivalves, and some specialized gastropods.
Various bivalves (e.g., cockles) and snails may perform rapid twists or jumps through violent flexion of the foot. Buoyancy floating and jet propulsion are found in cephalopods; floating is also known in gastropods, and swimming of a different kind is practiced by some opisthobranch and prosobranch gastropods as well as in scallops and related bivalves. Octopods use their arms to crawl or even to swim or float with the help of the body skin interconnecting the arms (interbrachiate web). Some bivalve groups bore into hard surfaces by secreting strong chemicals that dissolve the substrate or by drilling, using the shell and radula. A sedentary (sessile) way of life has been adopted by many bivalves and some gastropods.

[B]Locomotion in gastropoda:[/B]
the typical foot of a gastropod is a large flat creeping sole similar to the foot design of the ancestral mollusc. It has become adapted for locomotion over a variety of surfaces.

[B]Foot & Locomotion in scaphopoda: [/B]
The foot extends from the larger end of the shell and is spade or cone shaped. It is projected downwards, so the animal can burrow with it. It also serves as a means of an anchor for the animal. Finally, by the contracting and expanding motions, the foot also keeps water passing in and out of the posterior half of the mantle cavity which in turn causes blood circulation.

[B]Foot & Locomotion in apalcophora:[/B]

The foot is either virtually absent or vestigial: a simple ventral fold: It is much reduced and has become just a tiny median ventral ridge lying in a small longitudinal groove. This means that the Aplacophora have no viable means of locomotion.

[B]Foot & Locomotion in monopalcophora:[/B]
Monoplacophorans possess a foot, round in outline and not very muscular, which is responsible for locomotion. The muscular action is similar to that of the polyplacophorans.
Foot and Locomotion in polypalcophora
Chitons have a broad flat foot, which occupies most of the ventral surface of the animal. It serves both for locomotion and adhesion. Being very sedentary by nature, chitons, especially the older individuals, will stay in a very small area all their lives if an adequate food supply is available. For some species, this could be an area of as small as six square feet.
The foot secretes a small amount of mucous and propulsion is accomplished entirely by muscular contraction.
Both the foot and the girdle affect adhesion. Ordinarily, adhesion is accomplished just by means of the foot; however, when disturbed, the girdle clamps down on the hard substratum and the inner margin is raised. This creates a vacuum that enables the chiton to grip the surface with great tenacity. This is also the reason that chitons prefer smooth hard surfaces on which to live (rocks, shells of other molluscs, lobster traps and other sunken wood, anchors or other metal, etc. Interestingly enough, glass does not make a good substrate, because it is TOO smooth, which makes it difficult to get a truly secure grip.)
[B]Locomotion in cephalopoda:[/B]
All cephalopods swim by rapidly expelling water from their mantle cavity.

The squids are highly specialized at swimming, and bear a pair of posterior, lateral fins that act as stabilizers. The mantle contains both longitudinal and circular muscle fibers. On inhalation, the circular muscles relax and the longitudinal muscles contract enlarging the mantle cavity. This creates a suction and pulls in water through the space located between the anterior edge of the mantle and the head. When the mantle cavity is full the increasing pressure created by the water causes the circular muscles to contract that in turn closes the opening through which the water entered. The longitudinal muscles then contract and force the water through the ventral tubular funnel. The force of the expelled water as it leaves the funnel propels the animal in the opposite direction. This funnel is also quite mobile, allowing the animal to maneuver either backwards or forwards. Speed depends on how forcibly the water is expelled. Squid are able to hover or dart away very quickly. In normal swimming, the arms are stretched anteriorly and held closely together. Their fins are extended and they undulate gently. These fins wrap tightly against the body during rapid swimming. When escaping from predators such as large fish and dolphins, squid often jump right out of the water, and some species can travel surprising distances in these jumps, their lateral fins acting as little "wings"!
Nautilus can also swim with surprising speed. The process for this action is the same as that in the squid; however, the ejection of the water is produced when their body and funnel muscles contract rather than those of the mantle. Nautilus often rest on the bottom with their tentacles forming a stabilizing platform. Whether swimming or resting, their gas filled chambers keep their shell upright at all times. Scientists have yet to discover just how the nautilus regulates this gas production: perhaps you could be the one to discover this!!
The octopus is built for a more sedentary life style. Their body is globular and bag-like and it has no fins. The mantle edges are fused dorsally and laterally to the body walls producing a much smaller aperture into the mantle cavity. Octopus are able to swim as the squid do but in a jerkier fashion; however, they prefer to crawl about the rocks and crevices on the ocean floor. Their arms, which are studded with sucker discs, are used to pull the animal along or to anchor it to the substratum. Some octopods, the Vitreledonellidae, have returned to a swimming existence. Their arms have become webbed and look somewhat like an umbrella. These web arms are then used similarly to how a human swimmer would use his arms while doing the breaststroke.

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