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Nemertines are traditionally viewed as acoelomates with blood vascular systems and a specialized cavity housing an eversible proboscis, or, as coelomates with modified coelomic cavities forming the rhynchocoel, lateral vessels and gonadal sacs. Comparative ultrastructural data such as cell polarity, the occurrence of podocytes and mesodermal cells with rudimentary cilia for proboscis, rhynchocoel, lateral and dorsal vessels, and gonadal linings are consistent with their interpretation as modified coelomic cavities as found, analogously, in the higher leeches. These and additional data regarding musculature, connective tissue, glia-like cells and gametes for 13 species affirm that nemertines are intermediate between full-blown coelomate and acoelomate grades of organization. A comparison of ultrastructural details, however, indicates that their acoelomate attributes evolved secondarily from a more typical coelomate condition.

Along a line of the four nemertean orders, Archi-, Palaeo-, Hetero-, and Hoplonemertea established by Iwata, formation of the foregut is described in detail on the basis of data hitherto recorded. The axiality of the egg, larva and adult worm of a species may change as a function of foregut morphogenesis in the embryo. During foregut formation the mouth moves from the vegetal pole to the ventral side of the body. In Archinemertea, the larva has an angle of 60° formed by the long axis of the body and the foregut, while in Palaeonemertea the angle is 45°. This relationship is retained in the adult. The placement of the mouth thus obtained in the larva and adult worm seems to be important for the systematics of the Nemertea, especially for the Archinemertea and Palaeonemertea.

The feeding biology of nemerteans is reviewed, new information is presented, and the role of nemerteans in natural communities is discussed and evaluated. Most nemerteans are carnivorous, the majority feeding on live, often specific, prey, while some are scavengers. Macrophagous feeding is found in the Palaeonemertea, Heteronemertea and some of the Hoplonemertea; the more specialized suctorial feeding is limited to five of the hoplonemertean families, and suspension feeding (omnivorous diet) occurs only in the highly specialized symbiotic monogeneric Bdellonemertea. Feeding adaptations seem to be related to the nature and employment of the proboscis, the dilatability of the mouth, and the structure of the anterior part of the digestive tract. The transport of dissolved organic materials from seawater has been demonstrated, but its role in nemertean nutrition is just being contemplated. Present meager information on predation has shown nemerteans to have actually and potentially large effects on prey populations and thus on the communities in which they live. As the feeding biology of only a relative handful of species has been studied, much basic comparative research is needed before we can corroborate or refute our present ideas concerning the ecological roles of nemerteans.