Figure 2. The blood system of nemerteans. The basic plan is shown by the palaeonemertean Cephalothrix linearis (a), with various types of elaboration on this found in the heteronemerteans Cerebratulus lacteus (b) and Ramphogordius sanguineus (c), and the hoplonemerteans Amphiporus lactifloreus (d) and Prostoma graecense (e). (Modified from Gibson, 1972.)