Nemerteans have traditionally been thought of as acoelomates and the viscera are not enclosed in a body cavity. Recent studies however have revealed the presence of coelomic derivitives. There is an anterior brain and longitudinal lateral nerve cords. There is a circulatory system but accordint to recent intrepretation, at least some of the vessels formerly assumed to belong to the blood vascular system are in fact reduced coelomic spaces. This, plus the existence of the rhynchocoel, support the argument that the nemerteans are coelomate animals.
Excretion is via protonephridia.
The body wall is complex with numerous layers of muscles, epithelia, cavities, and connective tissue, which vary from group to group. Nemerteans can be studied in the laboratory using very large specimens which can be dissected or very small ones small prepared as wholemounts for observation with the compound microscope. Sufficient numbers of large dissectable individuals are rarely available, either alive or preserved, for use by an entire class and small species are usually preferred for that reason. A demonstration dissection of a large species, such as Cerebratulus , however, is a worthwlie inclusion in the laboratory and the following account is provided for that purpose. The genus Cerebratulus is represented by over 100 species, many of which are very large, some reaching 30 meters.