Order III. Metanemertini (= Hoplonemertea).—The brain and lateral nerve-cords lie in the parenchyma of the body internal to the muscle layers. The layers of the body-wall are similar to those of the Protonemertini. The mouth lies in front of the brain. The proboscis is armed. At the junction of the fore- and mid-gut a diverticulum is given off which projects forwards beneath the fore-gut and ends blindly in front.
Order IV. Heteronemertini (= Schizonemertea, and the genera Eupolia and Valencinia, placed provisionally by Hubrecht in the Palaeonemertea).—The body-wall consists of the following layers: epidermis, thick cutis, and an outer and an inner longitudinal muscle layer separated from one another by a circular muscle layer. The brain and lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. The mouth lies behind the brain. The proboscis is unarmed.
Development of the Nemertinea.—The development of the Palaeonemertea is at present not known: in the Schizonemertea a larval stage occurs; while in the Hoplonemertea the egg develops directly without undergoing any metamorphosis.
There are two forms of larva characteristic of the Schizonemertea, known respectively as Pilidium and the Type of Desor. The Pilidium is hatched early and leads a free-swimming existence, whereas the Type of Desor, though in many respects resembling it, never passes through the free-swimming phase.
Fig. 60.—Diagram of a Pilidium larva. (After Salensky.) c, Tuft of cilia; m, muscle-fibres; mo, mouth, seen through one of the lateral lobes; n, nerve-fibres; n.r, nerve-ring; n.g, nerve ganglion; oes, oesophagus; st, stomach.
The Pilidium (Fig. 60) is a helmet-shaped larva bearing a tuft or spike dorsally, and prolonged downwards laterally into a pair of lobes. The whole larva is covered with cilia, there being a specially strong band round its ventral surface. The dorsal spike is composed of a bunch of strongly developed cilia or of a long flagellum. The alimentary canal consists of a sac constricted into oesophageal and gastric regions (Fig. 60, oes and st). In this condition the larva swims about freely in the water. The helmet-shaped Pilidium-skin forms no part of the future Nemertine, the skin of which is developed as ingrowths from it; these meet one another and unite to form a complete covering round the alimentary canal; the larval skin is then cast off, and by a series of gradual steps the embryo develops into the adult.
Habits.—Nemertines are often found under stones between high- and low-water marks, lying on sandy or muddy bottoms. They are usually in the form of coiled masses, and are generally in a state of quiescence. Hence it is probable that their period of activity is during high-water, and that when left by the receding tide they subside into a resting condition.
The large kinds, such as Lineus marinus, seem to be always found living alone, but some of the smaller kinds, notably Tetrastemma dorsale and Prosorhochmus claparedii, have gregarious habits and live in masses, the coils of the different individuals being inextricably mixed.
Some species, such as Micrura purpurea, Amphiporus pulcher, and Cerebratulus angulatus, frequent empty bivalve shells, while Nemertines are often found in empty limpet shells adhering to rocks in tidal pools. Other smaller forms resort to no such definite protection, but live among seaweeds; some of these remain naked, while others secrete for themselves tubes of a membranous or gelatinous consistency. Borlasia elizabethae lives in a burrow of clay.