Reproductive System

The simplest mollusk reproductive system relies on external fertilization but there are more complex variations. All produce eggs, from which may emerge larvae, or miniature adults. Two gonads sit next to the coelom, a small cavity that surrounds the heart and shed ova or sperm into the coloem, from which the nephridia extract them and emit them into the mantle cavity. Mollusks that use such a system remain of one sex all their lives and use external fertilization. Some mollusks use internal fertilization and are hermaphrodites, both of these methods require more complex reproductive systems.

Mollusks that use such a system remain of one sex all their lives and use external fertilization. Some mollusks use internal fertilization and are hermaphrodites, both of these methods require more complex reproductive systems
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Respiration and Circulation

Organs for the respiration and circulation systems are located in the visceral mass. Not all mollusks live in water, some are on land. There are also all not fish, although some live in water. Gills assist the ones that reside in water. Aquatic mollusks such as snails, clams, and octopi typically breathe using gills inside their mantle cavity. Tubes call siphons bring water in and out of the body. Land snails and slugs do not have gills; they respire using a mantle cavity that has a large surface area lined with blood vessels. Diffusion occurs through moist skin of the mollusks; they prefer to live in moist conditions.

A mollusk has a heart, blood, and blood vessels. Oxygen and nutrients are carried to all parts of a mollusk's body by a circulatory system. Most have an open circulatory system- blood is collected form the gills, pumped through the heart, and released directly into spaces in the tissues. Then, it returns to the gills and then to the heart. These work well for slow-moving mollusks such as snails and clams because they demand for oxygen isn't very great. Cephalopods have a closed circulatory system. These faster moving mollusks like octopi and squid have a closed one because the system can transport blood through the animal's blood much more quickly. A blood filled space is called a hemocoel ("blood cavity"). The presence of discrete respiratory and circulatory systems has led to improved capacity for oxygen uptake and distribution, and hence an increase in body mass.
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Miller, Kenneth, and Josheph Levine. Biology. Upper Saddle River, New Jersey: Prentice Hall, 2002. 701-703. Print.

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