Unisegmental Vermes, with a ciliated ectoderm; a pair of cephalic ciliated pits or grooves; a protrnsible proboscis, placed dor sally to the oesophagus, opening above the mouth, and surrounded by nervous cerebral com-missures. Two main nervous cords extend from the cerebral ganglia, one on either side, to the posterior end of the body, and are sometimes united by a supra-anal commissure. There is a coelomic blood-vascular system, A pair of nephridia is situated in the anterior or oesophageal region. The generative organs are simple and paired, and extend in a series down either side of the body. The sexes are as a rule separate. Development takes place with or without metamorphosis. Mostly marine.

The body is frequently brilliantly coloured. It is usually elongated, somewhat flattened or cylindrical in section, and endowed with marvellous contractility. The transparent pelagic Pelagonemertes Rollestoni and the parasitic Malacobdella are short and broad forms. The former has a posterior ventral sucker.

1 Plate has recently proposed to divide the class into two groups, the Aductifera ( = Philodinadae) and the Ductifera. In the former there are two ovaries but no oviducts. The trochal apparatus retains the primitive form of two rings, one behind the other; the posterior styles are 4-6 in number, and the four cement glands open on their apices; the nervous system has no lateral antennae, etc. The Ductifera have a single ovary (germarium and vitellarium) and an oviduct; the trochal apparatus is modified; the terminal styles are never more than two; there are one or two dorsal and two lateral antennae, etc. See Z. W. Z. xliii, 1886, p. 233.

There is a delicate cuticle, which appears to bear the short numerous cilia; a cellular ectoderm, in which the cells are of different lengths; a basement membrane, and a cutis of connective tissue. The basement membrane in Schizonemertea lies between the ectoderm and the cutis, which is largely pervaded by longitudinal muscle fibres; whereas in the Palaeo- and Hoplonemertea it lies between the cutis and the muscular layers of the body. Many of the ectoderm-cells are glandular, and the gland-cells extend into the cutis. They secrete a plentiful mucus, with which are intermixed minute highly refracting rods. The animals are enabled to creep along the surface of water by means of this mucous exudation, and many, especially those which inhabit mud or sand, protect themselves by a coat or tube of it1. The muscular layers of the body are arranged in one of three ways. In the Palaeonemertean genera Carinella and Cephalothrix there is an outer circular, a median longitudinal, and an inner circular layer2. In the Palaeonemertean genera Polia and Valencinia and the Schizonemertea there is an external longitudinal layer of considerable thickness, a median circular and an internal longitudinal layer, whilst the Hoplonemertea possess only an external longitudinal and an internal circular layer.

New layers seem to appear in individual specimens of large size, and variations in the mode of preparation cause differences of appearance (Hubrecht). The muscular tissue of Borlasia is red-coloured, but the source of the colour is not known. There is a plentiful nucleated and partly fibrous connective tissue between the muscle layers and muscle fibres, which appears also to fill the central region of the body, so that there is no coelome3. Bands of muscular fibres are disposed dorso-ventrally in the region of the intestine, passing between successive lateral intestinal coeca. Hence the body appears to be divided internally into a series of segments.

1The mucus is remarkable for possessing in some instances an acid reaction, in others an alkaline. See Mcintosh, Monograph of British Annelids, pt. i.; Nemerteans, p. 45.

2 Pelagonemertes and Monopora are stated to possess an outer circular and an inner longitudinal coat.

3The coelome of the larval Lineus obscurus is an archicoele, and is simply a persistent blasto-coele or segmentation cavity. So far as it persists in the adult, it is represented by the cavity of the proboscis sheath and the blood-vascular system (Hubrecht). In the Pilidium, on the contrary, the mesoblast eventually forms two solid masses in the head and two in the trunk; the former develope some irregular spaces, the future blood lacunae; the latter splits into a somatic and splanchnic layer. But the cavity between the two is eventually broken up by the growth of cell-processes. In the early stages of Pilidium the mesoblast is chiefly represented by branched cells imbedded in a gelatinous matrix between the epi- and hypo-blast, in which they are freely moveable. These cells appear to give origin eventually to the permanent mesoblast (Salensky).

The nervous system consists of a right and left cephalic ganglion connected inter se by two commissures, one thick, below the proboscis and its sheath, the other, thin, above them; they are prolonged backwards into two lateral nerve-cords. Both ganglia and cords consist of a peripheral layer of ganglion cells1 and a core of fibres. The ganglia are simple in Carinella, Cephalothrix, and Carinoma. In other Nemertea each ganglion is divisible into a ventral lobe in direct continuity with the lateral cords, and a dorsal lobe, from which a posterior or third lobe is partially separated in Polia, Valencinia, and Schizonemertea. This third lobe in Hoplonemertea is connected to the main dorsal lobe by nerves alone, and it lies sometimes behind and sometimes in front of it, e.g. in Oerstedia, Amphiporus lacti-fioreus, etc. The inner or postero-median side of the third lobe is formed of large cells different to those of the rest of the ganglion. They sometimes inclose a ciliated space. The cephalic ganglia give off various nerves, some to the apex of the head, a branch to the oesophagus, and from the upper proboscidian commissure a fine median nerve extending backwards in relation with the sheath of the proboscis.