The only external structures in some are the labial palps (flap like extensions of the mouth); in some groups, there are sensory tentacles and photoreceptors at the edge of the mantle.
The mantle cavity of bivalves is greatly expanded and now surrounds the rest of the body. Inside the mantle cavity of most are the large leaf-like ctenidia , each of which consists of a dorsal vascular axis from which hang an inner and outer gill plate, or demibranch, made up of numerous parallel gill filaments.
In many bivalves, the mantle margin has a sensory function and often possesses rows of parallel tentacles with tactile and chemosensory cells. In some species, the siphon tips may also possess such tentacles. Simple eyes or ocelli may be present in the mantle margin, sometimes remarkably well developed, as in the case of the scallop Pecten, with a cornea, lens and retina. Although scallop cans have complex eyes with a lens and retina, but most forms have no or very simple eyes. Even the eyes of scallops probably do not function to resolve images, but arranged to detect movement and changes in "shadowing".
There is no radula and most bivalves are filter feeders that may use their gill apparatus also to capture particles of food. For example in some, suspended particles passing through the ctenidial filaments are mechanically trapped by rows of laterofrontal cilia or cirri and transferred onto the frontal ciliary tracts of the filaments. Small particles are then carried in mucus by the frontal cilia to the dorsal or ventral margins of the ctenidia, where they are incorporated into compact mucus strings and transported to the mouth for ingestion via ciliated food tracts. Larger particles may simply ride along grooves to other parts of the ctenidia where they exist from the cavity. A mussel of about 1 year has the ability to pump between 2-3 litres of water an hour, an older individual can pump as much as 6-9 litres an hour.
Link to feeding animations.
In many filter feeding bivalves, an elongated rod of solidified mucus referred to as the crystalline style projects into the stomach from an associated sac. Cilia in the sac cause the style to rotate, winding in a stream of food-containing mucus from the mouth, and churning the stomach contents. This constant motion propels food particles into a sorting region at the rear of the stomach, which distributes smaller particles into the digestive glands, and heavier particles into the intestine.
In some mussels the labial palps do the sorting, before the food in moved into the mouth and down the digestive tract.
Other organs are similar to those of other mollusks although position may vary depending on the type of bivalve being examined.
Clam -------------------------------------------------------------------------------------------mussel
Most marine bivalves go through a trochophore stage before turning into a free-swimming veliger larva. This type of larva looks like a miniature bivalve with a row of cilia along the edge of the mantle. Freshwater species lack these stages. Instead, some go through a larval stage known as the glochidium. Rather than being free-swimming, a glochidium attaches to fish or other objects that will not be swept downstream. Glochidia can be serious pests of freshwater fish.
Many bivalves (such as clams or oysters) are used as food in places all over the world. But should you eat oysters only in months that spell with an r in them. http://health.howstuffworks.com/wellness/food-nutrition/facts/shellfish-r-months1.htm
Pearl oysters are used for commercial production of pearls. Pearls can also be cultivated by pearl farmers placing a single piece of grit, usually a piece of polished mussel shell, inside the oyster. In three to six years, the oyster will produce a perfect pearl. These pearls are not as valuable as natural pearls, but look exactly the same. In fact since the beginning of the 20th century when several researchers discovered how to produce artificial pearls, the cultured pearl market has far outgrown the natural pearl market. Natural pearls have become scarcer and scarcer and a necklace with only natural pearls can easily cost several hundred thousand (US) dollars. [8]
Bivalves can also cause economic damage. The glochidia larvae of some freshwater mussels can be serious parasites of fish. As stated before, some species attract predatory fish to their marsupial gills with a minnow-like mantle flap. When fish approach and bite at the minnow lure, glochidia are released to infect the gills.
Most bivalves are sedentary and many are adapted for burrowing into soft sediments using the laterally compressed foot. A few genera, e.g. Teredo, are capable of boring into wood, and some are even able to bore into rock, e.g. Zirphaea, Lithophaga.
Although most bivalves are sedentary, some, especially members of the scallop family, are able to swim actively over short distances, usually to escape potential predators.
Class Cephalopoda
(octopuses and squids)
Although the Cephalopoda still contains a number of living species, fossil evidence suggests that they were once far more abundant than they are today, and were probably the dominant animals of early seas.
Univalve conical shell
• Exclusively marine
• Predatory (possible planktonivores in ammonoids)
• Most sophisticated brains among invertebrates
• Camerate (chambered) shell: may be coiled or straight
• Beak
The Cephalopoda is an ancient and very successful group of the Mollusca. Cephalopods first appeared about 500 million years ago in the Upper Cambrian Period.Cephalopods have been among the dominant large predators in the ocean at various times in geological history. Two groups of cephalopods exist today. Nautiloidea only containwith a few species of the pearly nautilus, but the Coleoidea contains the squids, cuttlefishes, octopods and vampire squids, which is represented by about 700 species. Cephalopods are the most active of the molluscs and some squids rival fishes in their swimming speed. Although there are relatively few species of living cephalopods, they occupy a great variety of habitats in all of the world's oceans. Individual species are often very abundant and provide major targets for marine fisheries.
When cephalopods first evolved, the primary feature that separated them from other molluscs was a shell with buoyancy chambers. This allowed cephalopods to invade the pelagic realm where predators and competitors were few. Today, only the Nautiloidea retain these shells. http://vimeo.com/58632858.
The major change from the basic molluscan plan exhibited by the cephalopods is the alteration of much of the original foot into a series of large prehensile tentacles or arms at the anterior end which now surround the head. The mantle surrounds the visceral sac and possesses strong muscles required for contraction of the cavity and respiration. An opening in the mantle cavity serves as an inhalant aperture, whereas the funnel serves as the exhalent aperture. Water is taken into the mantle (body) cavity for respiration, through the opening between the mantle and head. Contraction of mantle muscles expels the water from the mantle cavity through the funnel (a small ventral tube,sometimes called a siphon) for propulsion and elimination of waste products, in addition to completion of the respiratory cycle.
All cephalopods have one pair of unciliated ctenidia within the mantle cavity, with the exception of Nautilus, which has two pairs of ctenidia. The movement of water over the ctenidia is controlled by muscular contractions of the funnel or mantle wall. An external shell is possessed only by the Nautiloidea. Ocotopoids lack shells altogether. A cephalopod is also characterized by a horny beak secreted by the walls of the buccal cavity, and a radula within the buccal cavity.
All cephalopods are carnivorous. The strong beak is at the entrance to the buccal cavity , on the floor of which lies the radula. There are two pairs of salivary glands , one of which may be poisonous. The digestive tract consists of three parts: esophagus , which may contain a crop; stomach , which mashes food; and caecum , where most digestion and absorption occur. The posterior portion of the caecum contains a diverticulum that serves as an ink gland, producing a suspension of melanin that can be expelled through the mantle cavity.
With the exception of Nautilus, cephalopods contain pigment-rich cells in the epidermis surrounded by cells containing contractile fibers. These cells, called chromatophores, are responsible for the ability of the cephalopods to change color and patterns accurately and rapidly in response to danger or emotion. Chromatophores may also be under hormonal control. When the contractile fibers are stimulated, they contract and expose a larger amount of color.
http://www.youtube.com/watch?v=vIwjKcrxOOE&feature=related
Locomotion in cephalopods is accomplished mainly by jet propulsion. To close its mantle completely, a squid fits two cartilaginous ridges on the mantle wall into two cartilaginous grooves on the opposite funnel wall; contraction of circular muscles around the mantle cavity then forces water out the funnel. The funnel can be aimed, allowing the animal to change its direction. Locomotion in other cephalopods can be accomplished by other means. Octopoids can use their arms to "walk."
Cephalopods have distinct sexes. A female typically possesses a single oviduct. A male produces spermatophores that it transfers to the female's genital pore by means of a specialized arm or tentacle. In some species, the specialized arm tip may be pinched off and left in the female's mantle cavity: this is known as the hectocotylus arm. Mating in some cephalopods includes courtship rituals that may consist of color changes, body movements, or combinations of both.
http://www.youtube.com/watch?v=02zvS_QdJhw
Cephalopods exhibit spiral cleavage, but they have no larval stage: their development is direct. Octopods typically tend their eggs until hatching.
Cephalopods possess well-developed nervous systems and complex sensory organs. The ganglia are large and close to each other, forming a large brain. Certain upper lobes within the brain serve as controls for memory and learning. Cephalopods also possess ganglia elsewhere within the mantle cavity linked to the brain by giant axons that are involved with muscular contraction. The eyes in Nautilus are primitive, but in other cephalopods are highly developed and resemble vertebrate eyes with a cornea, lens, retina, and iris. These eyes are capable of forming images and distinguishing colors. http://discovermagazine.com/2003/oct/feateye#.UnE2fSSJTlQ
http://www.thecephalopodpage.org/smarts.php
It is believed that cephalopods evolved from an ancient group of gastropods.