By Joseph Enea
BackGround
Mollusca is the second largest phylum in terms of species. It is made up of over 110,000 species; most of these species are aquatic. The species in this phylum live in diverse environments which can range from arctic seas to tropical streams to high mountainsides. Very few live in the desert; some are even parasitic. Species can also range in size from the size of the giant squid to the size of a microscopic organism. This phylum is defined by the presence of a shell; the species' shells are long-lasting. The word "mollusca" is derived from the Latin word Molluscus which means "soft of body" in English. Members of this phylum include squids, snails, slugs, limpets, mussels, etc. Mollusks: are bilaterally symmetrical, have more than two cell layers, miss a body cavity, posses a gut with a mouth and an anus, are changeable in form, have a pair of kidneys, have a nervous system which includes a circum-oesophagal ring, ganglia, and paired nerve cords, have a circulatory system, have ctenidial gills, can reproduce sexually, feed on a variety of materials, and live in many environments. The structure of mollusks differ from most other animals. The three basic parts of a mollusk are the foot, the mantle, and the visceral mass. The foot allows a mollusk to move. The mantle is a layer of tissue that covers the visceral mass and attaches the shell to a mollusk. The visceral mass contains all of a mollusk's body parts.
Keyhole Limpets
A limpet is a common name used to describe freshwater and saltwater snails. Limpets breathe by using gills called ctenidia. These gills are located in their mantle cavity. The gills perform their best when completely submerged in water. The process of breathing begins when water is trapped by the limpet in the mantle cavity. Next, the limpet loosens its hold on the rock it lives on just enough to let the air diffuse into the water, but not enough to lose the water it has taken in. This process acts as a "lung" for the limpet. After oxygen is extracted, the water is released and the process continues again.
Mussels
Mussels are similar to clams. Mussels are at a low point of evolution compared to other animals. However, they are very unique. Mussels breathe the way they eat, by filter-feeding. Mussels take in oxygen from the water and food particles by opening its shell. The water and food particles are absorbed through gills. The gills are located in a bilave mantle cavity. Mussels can filter 5 litres of water per hour. Water is inhaled through the ingestive siphon and is later exhaled through the egestive siphon. The water is pushed in by cilia, which is located in the mantle cavity.
Slugs
Before their evolution into the terrestrial gastropods they are today, slugs were aquatic animals which used gills to gain oxygen. Over time, their gills began to disappear. These gills were replaced by the ability to extract oxygen from the air. Slugs use a lung which evolved from the inner surface of the mantle cavity to breathe. The lung is composed of highly vascularized tissue area. The lung surface must always be kept wet, just like the lungs of a human. The water allows gas exchange to take place. Slugs use the pneumostome, a single hole usually located on the right side of the mantle, to extract oxygen. The pneumostome opens and closes according to a rhythm as air enters and exits.