Mollusca+-+Nikhil

Phylum: Mollusca




 * Subkingdom**: Lophotrochozoa
 * Phylum**: Mollusca
 * Classes**: Aplacophora, Bivalvia, Cephalopoda, Polyplacophora, Gastropoda, Monoplacophora, Pleistomollusca, Rostroconchia, Scaphopoda

Head-foot - Contains the motor and sensory organs Visceral mass - Contains digestive organs Mantle - Hangs over the visceral mass and secretes calcium to form a shell
 * Body Plan**: Mollusks all have a head-foot, a visceral mass, and a mantle.


 * Germ Layers**: Mollusks are tripoblastic, meaning they have the endoderm, mesoderm, and ectoderm germ layers.


 * Symmetry**: Bilateral


 * Coelomate**: Mollusks contain coelom which develops inside the mesoderm. The coelom's primary function is to provide a hydrostatic skeleton and to suspend organs.


 * Habitat**: Mollusks live in many habitats such as marine, freshwater, and terrestrial habitats.


 * Movement**: Land mollusks, such as slugs, move via muscular ripples across a flat sole called the foot. Aquatic mollusks, such as squids, generally move by water-propulsion.


 * Skeletal System**: Mollusks have shells made primarily out of hardened calcium and chitin.


 * Evolutionary Advancement**: Mollusks began as shelled animals. However, over time this shell was first internalized and then lost altogether. This evolutionary loss of the shell developed in both marine and terrestrial species. This adaptation gave mollusks the advantage of increased mobility when burrowing and while swimming.


 * Special Adaptations**: Mollusks have adapted over time in both their feeding habits as well as their basic structure. Mollusks originally had shells, but this was seen as an evolutionary disadvantage because terrestrial environments did not provide enough calcium in order to build a full shell. Also, the lack of a shell gave allowed for increased movement. Mollusks were once indiscriminate feeders. However, today mollusks in different environments have taken individual evolutionary paths to a wide variety of feeding patterns. Mollusks can be herbivorous grazers, predators, and indiscriminate feeders.


 * Feeding Patterns**: Early mollusks were indiscriminate feeders, but today the various classes of mollusks each have distinct feeding patterns. The polyplacophorans and gastropods are considered herbivorous grazers. On the other hand, cephalopods are predators that actively seek out their food. Bivalve mollusks are indiscriminate suspension feeders that prey on bacteria, protists, and diatoms.


 * Respiration**: Diffusion in mollusks occurs mainly along external gills, but also occurs on the surface of the skin.


 * Circulation**: Most mollusks have an open circulatory system. Cephalopods are the only class which have closed circulatory systems.
 * Excretion**: Water is reabsorbed from the coelom by the nephridia. Waste is then excreted through the anus.
 * Nervous System**: Mollusks have four main nerve cords. The pedal nerve cords send signals to the foot and the visceral nerve cords send signals to the internal organs. Species of mollusks that have brains generally have ones encircling the esophagus.


 * Reproduction**: Marine mollusks generally have a definite gender, but terrestrial mollusks produce both sperm and eggs. In mollusks, up to 100 eggs can be fertilized. These eggs usually take about 2-4 weeks to hatch.

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