External Characters.—A typical Nemertine possesses an elongated worm-like body (Fig. 49), which is usually thrown into numerous close coils (Fig. 48). In section it may be either round or more or less flattened, with the lateral edges in some cases quite thin and almost fin-like. One or two broad, flattened, and leaf-shaped forms are known, but such a condition is exceptional, and the forms in which it occurs have probably assumed it owing to the adoption of special modes of life.
In the ordinary forms the posterior end of the body is pointed either bluntly or sharply. The head is somewhat broader than the rest of the body, and often assumes a spatulate form. Eyes (Fig. 51, e) are usually present either in one or several pairs, or in symmetrically-arranged groups on each side of the head. The mouth (Fig. 58, m) is situated near the front end of the body on the ventral surface, and is usually rendered conspicuous by being surrounded by thick tumid lips. It varies in form from being slit-like to elliptical. At the anterior end of the body a small terminal pore occurs; this is the external opening of the proboscis (Fig. 51, p.p).
Nemertines are often very diversely and brilliantly coloured, the hues most commonly found being white, yellow, green, deep purple, and various shades of red and pink. The ventral surface is usually paler in colour than the dorsal, and the latter is often marked by longitudinal and transverse stripes (Fig. 59) in contrasting colours.
The whole animal is enveloped in a layer of mucus, which sometimes becomes hardened to form a tube, and this may be still further strengthened by an admixture of particles of sand or earth.
The body is capable to a great extent of contraction and extension, a Nemertine many inches long being apt, when irritated or alarmed, to contract itself to the length of not more than half an inch. Hence, unless the animal is kept and carefully watched, a very erroneous idea may be conceived as to its size.
Anatomy.—The body-wall consists of several layers (Fig. 52), which in a typical highly-developed Nemertine are as follows:—
1. An external epidermic layer (ep), consisting of ciliated cells, among which are placed numerous unicellular glands. These glands probably secrete the mucus in which the Nemertine is usually enveloped; their contents when in the body are very highly refracting. The epidermis rests on a basement membrane (b.m).
2. The two or three muscular layers, arranged as either an external circular and an internal longitudinal, or an inner and an outer circular separated by a longitudinal layer, or, as in the figure (c.m and l.m), two longitudinal separated by a circular layer.
Fig. 51.—Amphiporus lactifloreus Johnst., drawn from the living specimen, from the dorsal surface. Plymouth. × 2. e, Eyes; g, generative organs; n.g, nerve ganglion; p.p, proboscis pore; p, proboscis.