Nemertea

Nemertea, an important class of 'worms,' the members of which are mostly marine. They are unsegmented, covered with cilia, and often brightly coloured. Their habitat is usually in sand or mud, but many are able to swim, while a few are parasitic. In diet they seem to be mainly, if not wholly, carnivorous. The body is very extensile, and in Lineus maximus may measure 15 feet in length. Most of them break readily and even spontaneously, the fragments being in some cases able to form a fresh head and body. Among their remarkable characters may be noted the presence of a long protrusible offensive proboscis which lies quite apart from the gut in a special sheath along the back, and the occurrence of two curious ciliated pits, perhaps respiratory in function, opening on the sides of the head, and sometimes reaching as far inwards as the brain. Two nerve cords extend from the brain along the sides of the body, occasionally approaching one another ventrally or even dorsally. The sexes are separate, and there is frequently a remarkable metamorphosis in development. According to Professor Hubrecht, the nemerteans exhibit in the proboscis, its sheath, and the two head-slits distinct affinities with vertebrates.