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 {114}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. Nemertines are commonly dredged from a depth of six or eight fathoms. They may sometimes be found floating on the surface of the water, and some possess the power of swimming rapidly, propelling themselves by a lateral motion of the tail, the sides of which are in such cases prolonged into a thin fin-like edge. This mode of progression is usually adopted by those which frequent deep water. A pelagic Nemertine (_Pelagonemertes_) was discovered by Moseley near the southern verge of the South Australian current, being found in a trawl with deep-sea forms from a depth of 1800 fathoms. This animal was leaf-like in shape, bluntly pointed behind and rather square in front.