Nemerteans have traditionally been thought of as acoelomates for the viscera are not enclosed in a body cavity. Recent studies however have revealed the presence of coelomic derivitives and necessitate the reevaluation of the phylogenetic affinities of these animals. There is a circulatory system but according 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 and other evidence, support the argument that the nemerteans are coelomate animals. There is an anterior brain and longitudinal lateral nerve cords.
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 species which can be dissected or by use of very small species which can be 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 worthwhile inclusion in the laboratory. For the purpose of this study, these four genera are nearly identical anatomically and can be used interchangable.