Nemertines are common on the British coasts; about forty species have been recorded from this area. On turning over a stone on a sandy or muddy shore in a pool left by the receding tide, there may often be seen a coiled mass, having the appearance of a uniform slimy string twisted into a complicated knot. If it be carefully removed, the ends can generally be made out, one bluntly rounded and the other slightly tapering (Fig. 48, _a_ and _b_). Occasionally there may be seen attached to the blunter end a fine thread, which moves about freely. This thread may, by an instantaneous movement, be drawn into the body, no trace of its existence being left except at the tip of the head, where a small pore is visible; this is the orifice through which it was withdrawn. Shortly afterwards the thread may be again shot out, the process being instantaneous and often accomplished with {101}great force. This thread (Fig. 50, _p_) is the proboscis, a very important and characteristic organ in Nemertines. Most Nemertines are marine; they are mostly indifferent to climate and to the nature of the soil on which they live. A few forms live on land (e.g. _Tetrastemma agricola_,[124] _Geonemertes palaensis_,[125] and _G. chalicophora_[126]) or in fresh water (e.g. _Tetrastemma aquarum dulcium_[127] and _T. lacustre_[128]) in various parts of the globe. There are also parasitic forms; the best known of which is _Malacobdella_.[129] A pelagic form, _Pelagonemertes_,[130] has been described by Moseley. [Illustration: FIG. 50.—Side view of head of _Cerebratulus_ (_Micrura_) _tristis_ Hubr., showing the everted proboscis. Naples. × 2. Drawn from a spirit specimen. _c.s_, Cephalic slit; _m_, mouth; _p_, proboscis.] 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 {102}small terminal pore occurs; this is the external opening of the proboscis (Fig. 51, _p.p_).