a) can reach quite large size (10-15 cm common; some to 30 m!)
b) share many traits with flatworms: multiciliated glandular epidermis, protonephridia, ladder-like nervous system, capable of asexual reproduction & regeneration, spiral cleavage
c) differ from flatworms: body is circular in cross-section, complete gut; unique blood-vascular system lined with endothelium; spectacular proboscis with rhynchocoel (a true coelom)