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 coelom, 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 reproduce sexually. Slugs and snails are hermaphrodites (possessing both male and female organs), but they must still mate to fertilize their eggs. Most aquatic mollusks lay eggs that hatch into small, free-swimming larvae called veliger.

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Most have an open circulatory system- blood is collected from 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.