The Phylum, Mollusca are the most successful of the animal phyla in terms of numbers of species. There are about 110,000 species known to science most of which are marine. They also exhibit an enormous range in size, from species which are almost microscopic to the largest of all invertebrates the giant squid. Most species cover a shell of some sort, these shells are long lasting and have been collected by human beings for thousands of years, some of these shells, and the pearls which come from oysters, which are also mollusks may be among the earliest forms of money.
Three classes of the phlyum Mollusca, in terms of the skeletal
system are Bivalvia, Cephalopoda, Scaphopoda
system are Bivalvia, Cephalopoda, Scaphopoda
Bivalvia
Bivalve shells vary greatly in shape; some are globular, others flattened, while others are longer. The shipworms of the family Teredinidae have great long bodies, but the shell valves are much reduced and restricted to the anterior end of the body. They function as burrowing organs that allow the animal to dig tunnels through wood.The shell is composed of two calcareous valves held together by a ligament. The valves are made of either calcite, or both calcite and aragonite, usually with the aragonite forming an inner, nacreous layer. The outermost layer is the periostracum, composed of a horny organic substance. This is usually olive or brown and is easily broken down. The shell is added to in two ways; the valves grow larger when more material is cut by the mantle at the margin and there is a gradual thickening of the valves throughout the animal's life.
The beak is located in the dorsal region of bivalves. The foot sticks out from the anterior of the shell, while the front of the shell is the region of siphon protrusion (at least in some bivalves). The plane between the two valves is the plane of symmetry which separates the left from right valve. Although this symmetry is carried in most bivalves, the Asymmetry corresponds to the living habit of the beasts. Those who have lost their original bilateral symmetry between the valves commonly live with their plane of the joint not perpendicular to the sediment surface.
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CephalopodaCephalopods, as the name implies, have muscular attachments extending from their heads and surrounding their mouths. These are used in feeding and even reproduction. In coleoids they number from eight or ten. Decapods such as cuttlefish and squid have five pairs. The longer two (tentacles), are actively involved in capturing prey. In giant squid they may reach a length of 8 meters. The shorter four pairs( arms), are involved in holding and shape the capture organism.Octopods only have four pairs of sucker-coated arms,though abnormalities can change the number of arms .The tentacle consists of a thick central nerve cord surrounded by circular and radial muscles. Because the volume of the tentacle contracts with circular muscles, it decreases the radius and allows rapid increase in length. .
The size of the tentacle is related to the size of it's buccal cavity; larger, stronger tentacles can hold prey as small bites are taken from it.Smaller tentacles prey is swallowed whole, so the mouth cavity must be larger. |
ScaphopodaThe aragonitite shells of scaphopods are pointed and curved in a flat spiral way, and they are usually whitish in color.
Because of these characteristics, the shell somewhat resembles a mini elephant's tusk. However, unlike an elephant's tusk, the shells of these molluscs are hollow and open at both ends. The opening at the larger end is the main opening of the shell. The smaller opening is known as the apical aperture. Some tusk shells 4 to 6 centimeters however, a few species reach 15 centimeters in length. The mantle of a scaphopod is entirely within the shell. The foot extends from the larger end of the shell. The scaphopod positions itself head down in the substrate, with the open end of the shell projecting up into the water. Water enters the mantle cavity through the apical aperture, and is drifted along the body surface by cilia. There are no gills; the entire surface of the mantle cavity absorbs oxygen from the water. A number of tentacles around the foot, called captacula, sift through the sediment and attach onto bits of food, which they then carry to the mouth. The mouth has a tongue-like band that breaks the bit into smaller pieces for digestion. |