This class is that of the ribbon worms or nemertina. There are about five hundred known species—perhaps as many more are unknown. Still near the bottom of animal life, they represent revolutionary advances from the lowest of worms, the planarians, with which they share many characteristics. They have evolved integrated brains and nervous systems. They have, for the most part, taken on a true worm shape. They have acquired weapons and, in some cases, arsenals of weapons. They have eyes that see. They have a digestive system, a mouth near the front of the body, and closed blood system through which flows a liquid which usually is colorless as water. Perhaps they hear. At the top of the head in certain species there is a group of cells with hairs and bristles which may constitute an organ of taste. Along the way of achieving these advances they have given up a little freedom and a little immortality for a little more efficiency.
Those best-known are inhabitants of sea shores, especially the Atlantic coasts of North America and Europe. They live under rocks, in abandoned mollusk shells, in windrowed masses of sea weed, in thin, parchment-like tubes which they secrete from their own skin. Their general appearance is that of a tangled mass of slimy string, but some members of the family have among the most brilliant color patterns known in nature.
The most conspicuous organ of these primitive worms is the proboscis, a hollow string which is shot out with great speed and force from the front end of the usually cylindrical body. At the end of the string, in several groups, is a sharp-pointed, barbed spear-like stylet with which the prey, usually some minute water animal, is speared. The victim then is drawn back into the mouth by the attached hollow thread. Some groups have no stylets. The thread, upon which is a mucilage-like mucous, is used like a lassoo and coiled tightly around the prey.
The proboscis is associated so closely with the brain that, like the retina of the eye, it has sometimes been considered an extension of it. The thread often is as long as the worm itself. It is shot out with such force that is frequently breaks off and continues to lead an independent life for a few hours. A new proboscis always develops.
When coiled, the proboscis rests in the center of the two-lobed brain. It is continuously shot out and pulled in and probes the water around it. Presumably at first it was an extremely sensitive sensory organ by which the brain was kept aware of its surroundings. The attached stylet, an offensive weapon, was a later development.
In a few cases the thread carries a multitude of unattached barbed points, a sort of machine-gun arrangement, which can be hurled in all directions in the hope of hitting something. It also carries tiny hooks by which it can be attached to some object. By means of the attached line the worm pulls itself forward over beach or sea bottom, its ordinary means of locomotion. It is also able, however, to glide like a planarian and to swim.
Nemertinea breath through the walls of the oesophagus, or gullet. When the tide comes in, shore-dwelling species rapidly swallow and eject mouthfuls of salt water. Oxygen to purify the blood is obtained from the water. The blood circulates in two or three vessels. It is a colorless plasma in which float both green and red corpuscles.
There is little knowledge as to the precise nature of the nemertinean sensory organs. There are, however, nerve cells in all parts of the body and the animal is quick to respond to any irritation, especially to any chemical change in the water. With an intense stimulus the body is contracted violently, twisted, and even torn apart. Even a headless specimen will move toward food placed nearby. A severed head may continue to creep restlessly for several hours. The headless body moves only when stimulated. With most mud-dwelling species it is difficult to secure an entire specimen. The slender, fragile body is likely to break into many fragments when disturbed. Quite commonly, even without any particular disturbance, a large worm will break up into a dozen or more pieces. Each becomes a small, new animal. Some regenerating fragments secrete disks of mucous and form cradles, in which they may remain for months while new organs are being formed. Eventually the disk ruptures and the new worm emerges. There is a specific tendency in some species thus to reproduce during warm weather, with a brief period of sexual reproduction during the cold months.
These worms are extremely tenacious of life. Even without food they may live as long as a year in the proper environment. Ordinarily they are quite voracious animals. They eat earthworms, other sea worms, small mollusks—almost anything soft-bodied which the eternally active proboscis can bring to the mouth. There it is sucked into the digestive tract. The digestive process is very rapid. Some species have distensible mouths. Like snakes, they can devour animals bigger than themselves. Some are cannibals. When times are hard they can, like planarians, absorb themselves. A case has been known where a nemertean digested all but a twentieth of its own body in a few months, apparently without any ill effects. The lost tissues were restored as soon as food again was available.