The human body, being on a much larger scale, takes more muscle to support, leaving us with comparatively little strength for lifting. Also known as water bears or moss piglets, these critters can survive a wide array of extreme conditions, including radiation, the vacuum of outer space, and temperatures all the way from degrees Celsius degrees Fahrenheit to over degrees Celsius degrees Fahrenheit. In the past, scientists believed this type of abstract cognitive skill to be unique to humans and other hominids. It has no feet, yet moves about as it pleases, and with incredible efficiency.
Etymology[ edit ] The taxonomic term Bivalvia was first used by Linnaeus in the 10th edition of his Systema Naturae in to refer to animals having shells composed of two valves. The name "bivalve" is derived from the Latin bis, meaning "two", and valvae, meaning "leaves of a door".
Some, such as the cockleshave shells that are nearly globular; cockles can jump by bending and straightening their foot. Others, such as the razor clamsare burrowing specialists with elongated shells and a powerful foot adapted for rapid digging.
The shipwormsin the family Teredinidae have greatly elongated bodies, but their shell valves are much reduced and restricted to the anterior end of the body, where they function as scraping organs that permit the animal to dig tunnels through wood. Bivalve shell Near the hinge of the shell is the umbooften a rounded, knob-like protuberance usually surrounding the beak.
The umbo generally and the beak specifically represent the oldest portion of the shell, with extra material gradually being laid down along the margins on the opposite sides.
The hinge point or line is the dorsal region of the shell, and the lower, curved margin is the ventral region. The anterior or front of the shell is where the byssus when present and foot are located, and the posterior of the shell is where the siphons are located.
In bivalves, the mantle lobes secrete the valves, and the mantle crest secretes the whole hinge mechanism consisting of ligamentbyssus threads where presentand teeth. This line known as the pallial line exists because, parallel to the opening edge of the bivalve's shell, the mantle is attached to the shell by a continuous narrow row of minute mantle retractor muscles.
The function of these small muscles is to pull the loose edge of the mantle up out of harm's way when this is necessary because of minor predation attempts.
In many bivalves, the mantle edges fuse at the posterior end of the shell to form two siphonsthrough one of which water is inhaled, and the other expelled, for respiration and suspension feeding. This is visible on the inside of the valve as an indentation on the pallial line which is known as the pallial sinus.
The valves are made of either calciteas is the case in oysters, or both calcite and aragonite. Sometimes, the aragonite forms an inner, nacreous layer, as is the case in the order Pterioida. In other taxaalternate layers of calcite and aragonite are laid down.
The periostracum is secreted in the groove between the outer and middle layers of the mantle, and is usually olive or brown in colour and easily abraded.
Although the sometimes faint concentric rings on the exterior of a valve are commonly described as "growth rings" or "growth lines", a more accurate method for determining the age of a shell is by cutting a cross section through it and examining the incremental growth bands.
Use of this technique has changed views on the longevity of many bivalves. For example, the soft-shell clam Mya arenaria was thought to be short-lived, but has now been shown to have a lifespan of at least 28 years. In different groups of bivalves, the ligament may be internal or external in position.
The main function of the ligament as well as joining the valves together is to passively cause the shell to open. The shell is actively closed using the adductor muscle or muscles which are attached to the inner surface of both valves.
The position of the muscles is often clearly visible on the inside of empty valves as circular or oval muscle scars.
Along the hinge line of the shell are, in most cases, a number of hinge teeth which prevent the valves from moving laterally relative to one another.
The arrangement of these teeth is often important in identifying bivalves. The animals have no brain ; the nervous system consists of a nerve network and a series of paired ganglia. In all but the most primitive bivalves, two cerebropleural ganglia are on either side of the oesophagus.
The cerebral ganglia control the sensory organs, while the pleural ganglia supply nerves to the mantle cavity. The pedal ganglia, which control the foot, are at its base, and the visceral ganglia, which can be quite large in swimming bivalves, are under the posterior adductor muscle.
Bivalves with long siphons may also have siphonal ganglia to control them. The organs are usually mechanoreceptors or chemoreceptorsin some cases located on short tentacles. The chemoreceptor cells taste the water and are sensitive to touch.
They are typically found near the siphons, but in some species, they fringe the entire mantle cavity. Each statocyst consists of a small sac lined with sensory cilia that detect the movement of a mineral mass, a statolith, under gravity.
These consist of a pit of photosensory cells and a lens.
These strong muscles connect the two valves and contract to close the shell. They work in opposition to the ligament which tends to pull the valves apart. In file shells that can swim by flapping their valves, a single, central adductor muscle occurs.
The paired pedal protractor and retractor muscles operate the animal's foot.This lesson is on the respiratory system of organisms in the phylum chordata. In this lesson we'll go over the types of animals in this phylum and.
Effects of pollutants on the aquatic environment Introduction. We can all relate to water. We know we need it to survive – indeed, the early great civilisations of Egypt and Mesopotamia were centred on river valleys where there was a plentiful supply of fresh, clean water.
Gill, in biology, type of respiratory organ found in many aquatic animals, including a number of worms, nearly all mollusks and crustaceans, some insect larvae, all fishes, and a few amphibians. The gill consists of branched or feathery tissue richly supplied with blood vessels, especially near the gill surface, facilitating the exchange of oxygen and .
The use of the Jacobson’s organ is most obvious in snakes. If a strong odour or vibration stimulates a snake, its tongue is flicked in and out rapidly.
With each retraction, the forked tip touches the roof of the mouth near the opening of the Jacobson’s organ, transferring any odour particles adhering to the tongue. Etymology. The taxonomic term Bivalvia was first used by Linnaeus in the 10th edition of his Systema Naturae in to refer to animals having shells composed of two valves.
More recently, the class was known as Pelecypoda, meaning "axe-foot" (based on the shape of the foot of the animal when extended).The name "bivalve" is derived from the Latin bis, meaning "two", and valvae, meaning. In the previous chapters, the sources of pollution, the general responses of fish to natural and man-made changes in the aquatic environment, and to some extent their interactions, were considered in outline.