Nemertines for the most part closely resemble one another in all essential points, though they differ considerably in size, colour, and external details. They vary in length from less than an inch to thirty yards, this extreme size being attained by Lineus marinus.
Fig. 48.—Lineus marinus Mont., from the living specimen in the coiled condition. Plymouth. × 1. a, Anterior end; b, posterior end.
Fig. 49.—L. marinus, from the same specimen as Fig. 48, in the expanded condition. a, Anterior end; b, posterior end.
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being left except at the tip of the head, where a small pore is visible; this is the orifice through which it was withdrawn.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 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.
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.