Phylum Mollusca
Structures: They are invertebrate animals. They are highly diverse in both size, anatomical structure, behavior and in habitat. Molluscs have shells that are formed by glandular cells in the mantle. The shell is composed of of an outer, prismatic layer containing densely packed cells of calcareous materials secreted by the end of the mantle. The inner layer is made of thin, laminated plates of calcareous materials laid down by the entire mantle surface.
Body Covering/Support: They posses a flexible body wall, that surrounds a body cavity containing the internal organs. The wall, which differentiates in the species, forms the structure called the mantle. The mantle is attached to the top of the body and surrounds it like a tent and the shell is formed on the outside of the mantle. On the underside of the body the wall is stretched out to form the muscular mass called the foot. The foot carries a pair of statocysts, which act like balance sensors. In gastropods the foot secretes mucus that aids in movement. The foot also acts like a sucker which allows the animal to attach to hard surfaces and the vertical muscles clammp the shell down on it. |
Nutrition: they use intracellular digestion to function. The have mouth with tongues (radulae), that bear many rows of chitinous teeth, which are replaced from the rear as they wear out. Their mouths contain glands that produce mucus, to which the food sticks. Then the mucus is driven to the stomach by beating cilia, so it forms a long string called the “food string”. Another group of cilia sorts out the particles. It sends smaller particles, minerals, to the prostyle where they are eventually excreted. The bigger particles , the food, are sent to the stomach to be digested.
Circulation of Nutrients: Molluscs have open circulatory systems. Their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is the hemocoel through which blood and coelomic fluids flow through and it encloses most of the other internal organs. The heart consists of one or more pairs of atria, which receive oxygenated blood from the gills and pump it to the ventricle which pumps it to the aorta, which opens into the hemocoel. The atria also functions as a part of the excretory system as it filters waste out of the blood and dumps them into the coelom as urine. |
Respiration: Most of them only have one pair of gills or even one gill that are feather shaped. Tey divide the mantle cavity sothe water enter near the bottom and exists near the top. If the osphradia detects noxious chemicals or sediment entering the mantle cavity, the gills cilia sops beating until the intruders have passed. Each gill has an incoming blood vessel that connects to the hemocoel and outgoing into the heart.
Nervous System: They have two pairs of main nerve cords organized around number of paired ganglia, which are the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia are linked by large bundles of nerves. There are three pairs of ganglia cerebral, pedal and visceral, where the visceral is the most important and largest because it is the center of thinking. |
Reproduction: They reproduce sexually and sometimes the process can be simple or complex. They develop eggs that fertilize and develop into a swimming form called a trochophore larvae, which is seen in the development of annelids. This organism then becomes a veliger larvae, differentiating according to the class and species, or becomes a miniature adult.
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Polyplacophora: These are the chitons or sometimes known as the sea cradles. They have shells composed of 8 separate shell plates or valves. The shell provides them with protection and allows them flex upwards when needed for locomotion over uneven surfaces and also allows them to roll into a ball when dislodged from rocks. They live worldwide, from cold waters to the tropics. They live on hard surfaces like under rocks.
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Gastropoda: these are the snails and the slugs. Most of them are hermaphrodites. They are herbivores and scavengers feeding on fungi, dead material and plant matter. They live under logs and rocks or in leaf litter.
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Bivalvia: These are the clams, oysters, cockles, scallops and the mussles. They are found in both freshwater and saltwater environments. They have no heads and they lack radula. The majority of them are filter feeders.
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Cephalopoda: These include the octopus, squids and cuttlefish. They are marine animals that have a bilateral symmetry, a prominent head and a set of arms or tentacles that are modified from their muscular foot. Their shell has been internalized or is absent.
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