Reproduction in mollusks can be sexual depending on the species. All species produce eggs; in most species, eggs are released from the female's body before fertilization. The simplest mollusk reproductive system relies on this kind of external fertilization.
There are more complex variations of fertilization. Some mollusk species have their eggs fertilized within the female's body and live young are born. Some mollusks that rely on internal fertilization are also hermaphroditic.
All eggs, when they are hatched, may turn into small, free-swimming larvae called veliger, or miniature adults. During reproduction two gonads sit next to the coelom and shed sperm into it (a small cavity surrounding the heart), from which the nephridia extract them and emit them into the mantle cavity.
There are more than 70,000 mollusk species, and they are categorized into different classes. The more common mollusks and the largest class are gastropods. They include slugs, snails, limpets, winkles and whelks. Another class is bivalves, which includes clams, oysters and scallops. Oysters can produce as many as 500 million eggs in a year. Bivalves are hermaphroditic or have separate sexes.The biggest class is the cephalopods, which squid, octopuses, cuttlefish and nautilus belong to. There are also smaller groups that include tusk shells and chitons (oval mollusks with jointed plates).