Most articulates have separate sexes. In addition, most species have no special breeding season, although some breed only at certain times of the year. There are usually four gonads, which are masses of developing gametes located beneath the mesothelium of the coelom. The ova or sperm are found in patches on the coelomic epithelium. They pass into the coelom and shell cavity through tubes in the segmental organs. In most species, large numbers of gametes are produced and discharged into the water, where fertilization takes place; in a few species, however, embryonic development takes place within the parent's shell, sometimes in a brood pouch.
Free-swimming larvae develop from the fertilized ova, propelling themselves through the water by rows of cilia on their three-lobed bodies. After a few days they sink to the bottom and undergo metamorphosis. At this point, the larvae have three developing regions: body, mantle, and pedicle lobes. No shell appears until the post-larval stage. The posterior lobe becomes elongated into a stalk, the middle lobe secretes a shell, and the remaining lobe forms the lophophore.
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