The Phylum Annelida (segmented worms) contains about 15,000 species of marine worms, earthworms and leeches ranging in size from less than a millimeter to over a meter!  Annelid characteristics include:  a metameric body, that is, one that is divided into repeated segments called metameres; a muscular body wall with an outer circular muscle layer and an inner longitudinal muscle layer; a cuticle secreted by epithelium on the outside; a well developed coelom divided by transverse septa; a closed circulatory system and complete digestive system (mouth, specialized tract, anus, etc.).  Gas exchange in annelids occurs through the skin, gills or lateral appendages called parapodia.  Excretion occurs via paired nephridia that are found in most body segments. The nervous system of annelids consists of a double ventral nerve cord, a pair of ganglia in each metamere along with lateral nerves and a bilobed "brain" consisting of a pair of dorsal, cerebral ganglia.  Sense organs of touch, taste, balance, photoreceptors and even eyes are found in some species.  In terms of reproduction, both monoecious and dioecious forms exist; the larva (if present ) is a trochophore similar to the one found in some molluscs.