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. [Illustration: 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.] 3. A fairly thick connective-tissue layer often found between the epidermis and the muscles, into which latter it gradually merges (_s.t_). {103}THE DIGESTIVE SYSTEM.—The mouth is placed on the ventral surface near the anterior end of the body (Figs. 53, 58, _m_). It leads into a straight oesophagus (Fig. 53, _oes_), whence passes off the intestine (_int_), which is continued as a straight non-convoluted tube to the anus (_a_), situated terminally at the posterior end of the body. The intestine is thrown out throughout the greater part of its course into paired lateral pouches. [Illustration: FIG. 52.—Diagrammatic transverse section of a Nemertine (Schizonemertea) through the middle region of the body. _b.m_, Basement membrane; _c.m_, circular muscle layer; _d.b_, dorsal blood-vessel; _ep_, epidermis; _g_, generative organs; _int_, intestine; _l.b_, lateral blood-vessel; _l.m_, longitudinal muscle layers; _n.c_, lateral nerve-cord; _n.l_, nerve plexus; _p_, proboscis; _p.s_, proboscis sheath; _s.t_, subcutaneous layer.]