Sources

http://www.sheppardsoftware.com/animalpages/animal-family-6.htm

http://www.oceaninn.com/guides/mollusks.htm

http://www.greenfacts.org/glossary/mno/mollusk.htm

http://www.bishopmuseum.org/research/natsci/waipiostudy/students/meet_the_critters/mollusks/introduced/Pomacea_canaliculata.htm

http://hometown.aol.com/ksmith9526/MolluskInformation.htm

www.mcwdn.org/Animals/Mollusks.html

www.sci.sdsu.edu/classes/bio201/mol.annel.htmlhttp://www.sheppardsoftware.com/animalpages/animal-family-6.htm

Anatomic Features of the Average Mollusk

Mucous gland: mucus-secreting organ
Digestive gland: organ that performs the functions of a liver and pancreas.
Crop: grinds up food before it is sent to gizzard to be further ground up.
Stomach: digestive cavity.
Shell: hard protective covering
Copulatory bursa: cavity related to the genital organs.
Heart: blood-pumping organ.
Pericardium: cavity related to the kidneys, situated around the heart.
Nephridium: organ that performs the functions of kidneys.
Genital opening: opening to the genital organs.
Excretory pore: opening through which a mollusk excretes its bodily wastes.
Gill: respiratory organ.
Pleural gland: tear-secreting organ
Pedal gland: organ related to the foot.
Mouth cavity: entrance to the digestive tract.
Tentacle: organ of touch
Eye: sight organ.
Cerebral ganglion: brain

What is a Mollusk

Mollusks are the large and diverse phylum Mollusca, which includes a variety of familiar animalsknown for their decorative shells or as seafood. These include tiny snails, clams, and abalone to larger organisms such as squid, cuttlefish and the octopus There are some 112,000 species within this phylum.

Interesting Facts

Cephalopods such as squid have eyes with lenses, retinas, and other features just like vertebrates. Some gastropods have a well-developed sense of smell and can find food in the water at a large distance. Predators can also be detected and so the mollusk can avoid them. Some mollusks exhibit complicated mating behavior. Some cephalopods possess an amazing ability to learn from experience.
The mollusk phylum is the second largest phylum in the animal kingdom, after the arthropods. Estimates of the number of mollusk species have sometimes exceeded 100,000.

Comparisons With Other Phylums

There are many simmilarities between the annelids and the mollusks. biologists believe that the two phyla are very closely related, branching off from a common ancestor during the precambrian period, about 600 million years ago. The mollusks are bilateral like the annelids. They also share a one-way digestive system characteristic. molluscs are the only phylum of ceolomate without segment bodies. they both have a hydrostatic skeleton for movement. Trochophores the larval stage of life for a mollusk, also appears in segmented worms, in the phylum annelida. .The octopus and the squid, 2 species of mollusk, have some of the most complex nervous systems, held by invertebrates. the giant squid is the world's largest invertebrate.

Habitat and Adaptations

One of the most widespread and successful forms of life, mollusca has managed to make a living in all but a few places. Mollusks can be found in every part of the oceans, from the deep trenches to the shallows, the arctic to the equator. They also adapted to live on land, though not as extensively and in water. Snails, as well as a smaller number of other mollusks, can be found in large numbers in the rainforest canopies, but can also be seen in your backyard. One land type which mollusks have not been able to invade yet are the deserts. Their skin is very moist and requires the presence of water, or else it dessicates. To stop this unpleasant death, land dwelling snail have a thick layer of skin at the opening of their shell which keeps moisture inside. The snails also produce a coat of slime or mucus which stops it from being dried out.

Reproduction

Mollusk reproduction is most commonly done externally, with the release of eggs and sperm into the environment resulting in the fertilization of the eggs. The larvae formed from this type of reproduction are usually free swimming, and either settle of the ocean floor or drift to mature. Some members of the phylum, such as tentacled mollusks and some types of snails, fertilize their eggs internally. Some mollusks are hermaphrodites. They can alternate between the production of male and female gametes, which gives the animal another potential mate... itself.

Mollusks produce a special kind of larvae known as a trochophore, which is translucent and free-swimming. They are round or pear-shaped, and are surrounded by a ring of cilia called the prototroch which propels the larvae through its environment. Also found on the trochophore, above the prototroch, is an ocellus (a simple eye) and a tuft of cilia. Below the belt of cilia are the mouth, stomach, anus, and solenocyte, which regulates internal salt-water balance. In some mollusks, marine snails and bivalves, the trochophore enters a second stage called the veliger before the animal if fully matured. In this stage, the larvae begins to develeop a foot and shell. Trochophores also appear in segmented worms, in the phylum annelida. This fact has led biologists to believe that the two phyla are related, and probably branched off from a common ancestor during the precambrian period, about 600 million years ago.

Important Vocabulary

Mantle - fold of outer skin lining shell or enclosed to create a cavity. excretes shell.

Gills - flaps of mucus covered membrane that take oxygen out of the water

Foot - a muscular foot, used for locomotion.

Shell - a hard protective surface enclosing many molluscs

Radula - Chitin enforced tongue used to scrape of food.

Visceral Mass - mass containing digestive, circulatory, respiratory, and nervous systems

Open Circulatory System - Blood does not always flows within blood vessels

Closed Circulatory System - Blood always flows within blood vessels

Filter Feeder - feeds my means of sucking in water, removing small particles for digestion

Respiration and Circulation

Since Mollusks have adapted to conquer both land and ocean habitats, different means to get Oxygen to the cells of the body have evolved. In Water-dwelling mollusks, gills used to take oxygen from the water can be found inside the mantle cavities. These gills are known as ctenidia or singularly, ctenidium. Landlubbing mollusks do not have ctenidia. Instead, their mantle is folded and contains many blood vessels. This structure is more similar to the human lung than the gills are. Inter-tidal mollusks are able to alternate between respiration in and out of water. Once in the bloodstream, the oxygen is carried throughout the mollusks' body by the heart. Some Species have on open circulatory system, meaning that blood does not always travel through special vessels. This style of Oxygen diffusion is adequete in sessile or slow mollusks like snails and clams, but more active groups of mollusks require a closed circulatory system. Examples of active mollusks with closed circulatory systems are squid, octopi, and cuttlefish.

Feeding, Digestion, and Extraction

Mollusca of the Bivalvia class are filter feeders. The most well known of these species is the clam. When a clam opens and closes its shell, it sucks water through the incurrent siphon, into its body, creating a current. This current travels through the gills of the clam. Many glands excrete mucous which coats the gills. while the clam respirates, tiny bits of food are caught in the mucus. This food filled mucous is then pushed along by cillia on the gills of the clam to the mouth where it is digested. After the food is digested, it is shot out of the excurrent siphon. The liquid waste is removed by the extractory gland and shot out of the siphon as well. This way of trapping food in mucous is quite similar to the way dirt particles are caught in the mucus of a human's nose, and then brought into the stomach for digestion.

Many molluscs feed using their radula. The radula is a chitin enforced tongue that is used to scrape algae, plant or animal matter off of the surface that the specimen is grazing over.

Cephalopods also feed using a radula but they also have a hard beak used to break open the shells of clams and crabs. Using their long tentacles they quickly snag their prey before pushing it towards their mouth. The beak then breaks open the shell and many digestive enzymes are excreted from glands around the mouth to digest the prey. The liquids are then sucked into the mouth, and the shell is dropped.

Anatomy

Mollusks are triploblastic protosomes which have evolved for life in both sea and land. Almost all Mollusks have four basic parts in common. These are the foot, shell, mantle, and visceral mass. The foot, despite being called a foot, is used for feeding in many species and contains the mouth. The muscular foot is also used for movement in most species, and it has evolved to be a burrowing, crawling, and prey capturing organ. The shell, which is secreted by cells is the mantle, is composed primarily of Calcium Carbonate, but also contains other organic compounds in its' many layers. Beneath the shell and mantle can be found the visceral mass. This part of the Mollusk contains the internal organs, which necessitates protection by the shell. An organ found only in Mollusca and nowhere else on the invertebrate family tree is the radula. It is a tongue like organ with many tiny sharp teeth running along it. The Radula has evolved to suit species with different feeding styles. In herbivores, it is used to scrape algae from rocks and in some carnivores, to drill through the hard shell of prey.

Classes

Gastropoda
There are approximately 40,000 living species of gastropoda. They can live in a terrestrial, marine, and freshwater environment. Most species in this class have a shell although some have an internal shell, or none at all. Some examples of this class are snails, slugs, and nudibranchs.

Bivalvia
Bivavlvia species all have 2 shell valves which is how they got their name. Most of this class consists of marine species, while a few are fresh water. Most of them filter feed plankton and other organic matter out of the water. Some species in this class are mussels, clams, and scallops.

Cephalopoda
This class contains the most intelligent, largest, and fastest swimming invertebrates. They all have tentacles, and swim using a jet propulsion system. Masters of disguise, some are capable of changing their colours to match the environment, and hiding in the smallest of nooks, and crannys. Some also have the abillity to squirt ink at an attacker to temporarily blind it to escape. Some species include the octopus, squid, cuttlefish, and nautilus.

Aplacophora
This class contains worm like molluscs that live mostly in deep marine waters. They all lack any eyes, tentacles, or shells.

Monoplacophora
Little is known about this class of mollusc. there are only 11 different living species that have been found. They live in very deep marine waters, and quite closely resemble a limpet.

Polyplacophora
The species in this class have seven or eight dorsal shell plates used for protection. They are mostly found in the intertidal zone stuck to rocks. An example of this species is the chiton.

Scaphopoda
This class contains molluscs that have a tusk like shell with an opening at both ends. One opening is buried in the sand while the other has many tentacles used for feeding. They are all marine species, and they live mostly in shallow to moderately deep waters.

Caudofoveata
Very little is known about this class of mollusc. They are small and wormlike, living with their head buried in the sand. There are only 70 discovered species.