User:Tonei/Notepad/Mussel

Introduction

Mussels are members of the phylum Mollusca, which covers soft-bodied invertebrates which are generally at least partly enclosed in a calcium carbonate shell. The phylum includes narrow-footed gliders (Solenogastres); chitons (Placophora); neopilinids (Tryblidia); limpets, snails, and slugs (Gastropoda); tusk shells (Scaphopoda); nautiluses, cuttlefish, squids and octopi (Cephalopoda); and of course the clams, mussels, scallops, oysters, cockles and mussels - the bivalves.

Digestion and Excretion

Mussels use a filter feeding method similar to that of the sponge. It consists of a complex stomach and associated structures, but the intestine is an otherwise simple one. Water enters the incurrent siphon, the first of two siphons in the digestive system. The incurrent siphon allows water in and out of the body through the gills. There are typically two phases of digestion - extracellular (in the stomach) and intracellular (in the digestive diverticula). In the extracellular phase, food particles are transported by cilia and broken down by enzymes (released by the crystalline style). The remaining particles are passed on into embayments (holding areas) and then to the digestive diverticula for intracellular digestion. Waste material is consolidated in the midgut and expelled as firm pellets from an anus opening into the excurrent siphon.

Metabolic wastes are disposed of through nephridia, which collect wastes from the coelom and pump them into the mantle cavity, where they are expelled in the excurrent siphon.

Nervous System

Mussels have very simple nervous systems, which reflects their sedentary lifestyle. Mussls have two cerebropleural ganglia on either side of the esophagus. There is also a pair pedal ganglia in the base of the foot, and a pair of visceral ganglia under the posterior adductor. Nerve fibres extend from the cerebropleural ganglia to the pedal and visceral ganglia. Mussels have no ocular sensors.

Reproduction The reproductive system of bivalves is very simple. It consists of paired gonads which open by separate gonopores into the suprabranchial chamber. The sexes are usually seperate, but some levels of hermaphroditism are not particularly uncommon. Eggs and sperm are shed into the sea for external fertilization, although some female bivalves intake sperm, which permits internal fertilization and brooding.

Circulation and Respiration

Mussels have three-chambered hearts, like those of frogs. They also have an open circulatory system like that of the grasshopper. This allows oxygen and nutrients to be carried directly to the organs and wastes to be carried from them through the blood. There are also specialized veins which carry blood directly to the gills, because of their substantial energy requirements. The gills are part of the mantle and consist of filamentous projections. They have very large surface areas ; however, the respiratory demands of these sedentary creatures is low. It is thought that in freshwater mussels, respiration probably takes place across the body and mantle as well, because they are also immersed in water.

Locomotion

Bivalves generally use their locomotive abilities sparingly, due to their low efficiency. The bivalve foot is blade-like (laterally compressed), and is pointed for digging. The anterior and posterior pedal retractors are the muscles mainly responsible for foot movement. These control the foot by retracting it and moving it back-and-forth. The foot is extended as blood is pumped into it, but a series of muscle fibres prevent it from overinflating. These have the added benefit of directing pedal extension and allowing for further mobility control.

Symmetry and Special Characteristics

Mussels are bilaterally symmetrical. Most mussels have a lifespan of 20 to 30 years, although some have been known to live over 100 years.

Resources

Bourquin, Avril. Man and Mollusc. Apr. 2005. 18 May 2005 <http://www.manandmollusc.net/>.

"bivalve." Encyclopædia Britannica. 2005. Encyclopædia Britannica Premium Service 17 May 2005 <http://www.britannica.com/eb/article?tocId=9015442>.

"mussel." Encyclopædia Britannica. 2005. Encyclopædia Britannica Premium Service 17 May 2005 <http://www.britannica.com/eb/article?tocId=9054463>.

"bivalve." Encyclopædia Britannica. 2005. Encyclopædia Britannica Premium Service 17 May 2005 <http://www.britannica.com/eb/article?tocId=9110325>.