Intestine straight, not branched; body elongated, rounded, or oval.
Intestine branched or arborescent; body flat and broad.
The Nemertida, or "Ribbon-worms," agree in most essential respects with the Planarida. They are distinguished, however, by their elongated, vermiform shape, by the presence of a distinct anus, by the possession of a distinct perivisceral cavity, by the absence of an external aperture to the water-vascular system of the adult, and by the fact that the sexes, with one or two exceptions, are distinct. The Nemertida further differ from the other Platyel-mia in possessing a pseudo-haemal system in addition to, and distinct from, the water-vascular system. The external surface of the body is richly ciliated, and is underlaid by a thick glandular cutis, beneath which are well-developed sub-cutaneous muscles. The digestive canal is ciliated internally, and consists of a muscular gullet, a sacculated stomach, and an intestine with a distinct anus. The nervous system consists of two large cephalic ganglia, united by a double commissure, and sending lateral cords backwards. The so-called "circulatory system " is composed of closed contractile vessels, sometimes containing a corpusculated fluid. "Along the median line of the dorsum lies a special muscular sheath, containing a complicated proboscis, and a highly organised corpuscular fluid, both the sheath and the proboscis passing between the commissures of the ganglia in front" (M'Intosh). The evertible and muscular sheath of the proboscis may be as long as the whole body, and the extremity of the latter may or may not be protected by one or more spines (fig. 118). The sexes are mostly in separate individuals, and the generative organs have the form of sacs placed between the muscular walls of the body and the digestive canal, and discharging their contents by lateral pores.
Reproduction takes place by the formation of true ova, by internal gemmation, or by transverse fission. In Nemertes, however, the egg gives rise to a larva, from which the adult is developed in a manner closely analogous to that described as characteristic of the Echinodermata. The larval form of Nemertes was described by Johannes Muller, under the name of Pilidium (fig. 117,4). It is "a small helmet-shaped larva, with a long fiagellum attached like a plume to the summit of the helmet, the edges and side-lobes of which are richly ciliated. A simple alimentary canal opens upon the under surface of the body between the lobes. In this condition the larva swims about freely ; but, after a while, a mass of formative matter appears on one side of the alimentary canal, and elongating gradually, takes on a worm-like figure. Eventually it grows round the alimentary canal, and appropriating it, detaches itself from the Pilidium as a Nemertid - provided with the characteristic proboscis, and the other organs of that group of Turbellaria " (Huxley). Whilst some Nemertids are thus developed by internal budding from a ciliated provisional larva or "nurse," others exhibit no such phenomena, but are directly developed into the adult form, without undergoing any striking metamorphosis.
Whilst the Nemerteans undoubtedly show very close affinities to the Planarians, they are also nearly related to the Annelida proper, both as regards their general form and their internal structure. One of the most remarkable of the links between these two groups is to be found in Balano-glossus, the systematic position of which is still uncertain. This extraordinary worm is found burrowing in the sand in the Mediterranean and along the eastern coasts of North America, ranging to depths of 2500 fathoms, and possesses a flattened worm-like body, terminating in front in a protrusible hollow proboscis. The external integument is ciliated, as in the Nemerteans, and there are no setiform organs of locomotion, such as are found in so many Annelides. A water-vascular system is present ; there is a well-developed alimentary canal; and the process of respiration is carried on by means of a singular branchial sac formed by a chitinous framework with ciliated apertures, somewhat similar to the "branchial sac" of an Ascidian ; whilst the sexes are in different individuals. The most remarkable point, however, about Balanoglossus is its mode of development. The larva is a free-swimming, barrel-shaped, ciliated body, which was originally discovered by Midler, and described by him under the name of Tornaria, as the young of some Star-fish. In spite of its close resemblance to the larva; of the Echinoderms, A. Agassiz has, however, succeeded in showing that T'ornaria is really the young of Balanogiossus, and that it is developed into the adult by a rapid change, not accompanied by any absorption, or casting off, of any portion of the original larva.
Fig. 118. - Morphology of Nemertida. Prorhyn-chus fluviatilis: o Mouth ; c Ciliated grooves (sense-organs ?); s Spine, attached behind to (p) the sac of the proboscis.; oe Gullet; g Gastric glands ; i Intestine ; ov Ovary. The proboscis in this form is very small. (After Gegenbaur.)
The Nemerteans are mostly marine in their distribution, a few forms inhabiting fresh water, and two (the Tetrastemmae of the Bermudas and Philippine Islands) being found in moist places on land. They are found from the arctic seas to the equator, most of them being littoral in their habits, though some live at considerable depths. Recently, Mr Moseley has described a peculiar group of Nemerteans under the name of Pelagonemertidae, which are pelagic in their habits. These oceanic forms have a broad, gelatinous, flattened body, and a ramified digestive tract, and thus make a near approach to the dendrocoelous Planarians. No certain remains of Nemertids are known to occur in the fossil state, though some obscure remains have been referred to this group.