Bilateria
Protostomata
Lophotrochozoa
Acoelomata
Defining characteristic: muscular eversible proboscis in a fluid-filled schizocoelous cavity.
Common name - ribbon worms. ~900 species. Name derived from the long muscular proboscis they can thrust out to grasp prey (Greek νεμέρτος - unerring one). Can vary in size from a few mm to several metres (up to 27m).
Most are benthic marine animals; some are commensal in gills of bivalves, crustaceans and ascidians; some species are planctonic in deep ocean waters, few terrestial (tropical moist areas) and freshwater.
Triploblastic, acoelomate, unsegmented. Ciliated externally, secret mucus through what they move; no hard cuticle or skeleton. Mesodermally-derived parenchyma as in flatworms.
Nervous system of cerebral ganglia and nervous cords, connected with connectives. Ganglia suround proboscis. Have sensory organs such as ciliated pits and ocelli.
Have protonephridia and flame cells.
Closed circulatory system of two main longitudinal vessels linked by transverse vessels. Blood vessels lined with mesoderm, what's unique for Nemertea. No heart or respiratory system. Pigmented blood cells - red, green, yellow or orange.
Complete gut. Internal organs surrounded by mesenchyme, no coelom. Digestion largely intracellular. Muscular body wall - internal longitudinal, outer circular.
Separate sexes, external fertilisation. Development - spiral cleavage. Direct development (juvenile looks lika a small adult). Pilidium larva with complex metamorphosis - only in order Heteronemertea.
Believed to be more closely related to coelomates than to flatworms. May be degenerate coelomates.
A whole site about nemertean anatomy:
http://nemertes.si.edu/Atlas/