14 NEMERTINEA 



geal and gastric regions (Fig. 60, oes and at}. 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 Linens marinus, seem to be always 

 found living alone, but some of the smaller kinds, notably Tetra- 

 stemma 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 gela- 

 tinous 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 (Pelagone- 

 mertes) 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. 



The power possessed by Nemertines of secreting mucus is 

 very great, their course being often traceable by the tracks which 

 they leave behind them Many of them glide along with great 

 rapidity, a mode of progression which is probably due to the 



