The smallest of these soft, elongated, mostly marine worms may be threadlike and only a fraction of an inch long. The giants of the group, however, are the longest, though certainly not among the largest, of invertebrates. Exactly how long it is difficult to say, for all the ribbon worms are highly elastic, and the really long ones stretch out, threadlike, for yards and yards — some say much more than 30 yards in Lineus longissimus, the blackish brown worm of the North Sea. The English call it the "bootlace worm." Modest length, not more than about 8 inches, is more usual. The body may be cylindrical, as in Lineus, though more often flattened on both sides or flattened below and convex above.
Bright colorings of orange, red, purple, or green, these mostly on the upper surfaces, may betray the worms to the eyes of naturalists scanning rocky crevices or overturned stones at low tide. More often the colors blend with red or green algae or other colorful growths among which the worms live. To find small nemerteans, collectors place masses of seaweed or of bryozoan colonies that resemble delicate seaweed in dishes of sea water and let the small worms creep out on the walls of the dishes, where they can easily be seen. Some worms are white or yellowish, others somber grays or browns, but many are handsomely patterned with strongly contrasting rings or longitudinal stripes or both. The front end is not set off as a distinct head, though the tip may be expanded and have colored markings, several or numerous eyes, and sensory grooves, which make it look superficially like a head. The rear end is more or less pointed.
Another common name, proboscis worm, less widely used, calls attention to the most distinctive feature of nemerteans. This is a long, extensible, tubular proboscis that can be shot out the front end with explosive force to grasp prey or discourage enemies. The proboscis coils about the prey, holding it firmly and entangling it in sticky mucus which may be irritating or even poisonous. The proboscis is also everted as a device for burrowing in sand or mud or for attaching to objects as an aid in creeping about. It can be made to evert by irritating the animal, by plunging it into fresh water, or by placing it in a small dish of sea water and cautiously adding alcohol, drop by drop. The accurate aim of the proboscis receives recognition in the technical name of the phylum, Nemertea, from a Greek word that means "unerring." In some of the commonest worms the tip of the proboscis is armed with a sharp spike or stylet, which pierces the prey, sometimes several times, before a toxic secretion is poured on. Worms may have two or more pouches with a reserve supply of stylets, so that replacement can be made quickly if the main one is damaged. When not in use the proboscis is sheathed in a muscular tube that lies above the digestive tract.
As it goes on mostly at night, feeding is not often observed. The favored food seems to be annelids, and these have been seen to be swallowed whole, making a prominent bulge in the thin, elastic body of the nemertean. Mollusks, crustaceans, and fishes are also eaten, though bigger prey may be sucked at, not downed in one piece. Undigested residues do not have to be cast out the mouth, for the nemerteans are the lowest animals that have an anus, a second open- ing to the digestive tract, which voids materials from the rear end of the animal. The ribbon worms are built much like flatworms, but aside from the anus they can boast another important improvement. They have contractile blood vessels. Waves of contraction in the strong muscles of the body wall also help to push blood and food along their respective tubes, and in a worm at rest it is these powerful muscular waves that are seen to pass along the body.
A few ribbon worms swim by undulations of the long body. The young and the smaller forms glide along, by means of beating cilia on the body surface, over a lubricating bed of secreted slime. In larger worms more use is made of muscular contractions for creeping. Some even spiral ahead at times by agile body contortions.
One may grasp several inches of a delicate, slimy nemertean and pull cautiously lest it break, yet have it slip from one's fingers and disappear down a crack in the rock. Worms that do break in escaping from would-be captors, human or animal, almost always replace a missing rear end; and certain species can regenerate a whole worm from any fragment that contains a portion of one of the lateral nerve cords. As in flatworms, the capacity for regeneration goes with the natural capacity of certain species for reproducing asexually by fragmentation of the body, especially during warm months. A large specimen of Lineus socialis, which lives gregariously under stones on the American Atlantic coast (or of Lineus vegetus on the west coast), may fragment into six to twenty or more pieces. After transforming into complete worms of smaller size, these grow again and later reproduce sexually. Most though not all ribbon worms are of separate sexes. The eggs are usually laid in gelatinous strings or masses, and the young hatch as juvenile worms. In some species of Lineus, in Cerebratulus, and in some of their relatives, the egg hatches as a gelatinous, helmet-shaped, free-swimming little larva, called a pilidium. It must feed on microscopic organisms and develop further before it takes on the structure of the adult.
For the most part, ribbon worms are bottom dwellers on temperate marine shores, where they burrow in mud or sand or creep about among rocks and seaweeds between tide marks or in shallow waters. Only a few burrow into the deep-sea bottom, sometimes at depths of forty-five hundred feet or more. Of some 570 described species, nearly 200 are found along the Atlantic or Mediterranean shores of Europe. About 100 live on the Pacific coast of North America, at least 18 of them identical or very similar to European species. The Atlantic coast of North America has few more than 50 known species, and W. R. Coe, the American authority on nemerteans, thought this was due to the cold arctic current that comes close to the coast as far south as Cape Hatteras, for many of the missing genera are warm-temperate forms. Almost 30 species are described from Japanese shores. In the open seas, chiefly the southern parts of the North Atlantic, there are nearly 60 gelatinous species that drift or swim slowly far below the surface. They have been brought up from depths ranging from six hundred to nine thousand feet, most from below three thousand feet. Nemerteans are less common in tropical or subtropical seas, but well rep- resented in arctic and antarctic waters, often by the familiar temperate genera: Lineus, Amphiporus, Cerebratulus, and Tetrastemma.
Perhaps the most cosmopolitan species is Lineus ruber, found from Siberia to South Africa. The slender, rounded body is 3 to 9 inches long; and different varieties are colored red, green, or brown, any of them difficult to see in natural surroundings, even when one has lifted the stone under which the worm lives. Fresh waters, especially in northern latitudes, harbor species of the genus Prostoma. What seems to be a single species, Prostoma rubrum, a slender reddish worm less than 1 inch long, can be found in pools and quiet streams in nearly all parts of the United States. It clings to the leaves of aquatic plants and feeds on minute crustaceans, nematodes, and turbellarians. In Europe this genus has also an eyeless variant that lives in caves.
Land nemerteans are all of the genus Geonemertes. The two best-known species are slender, pale in color, and not more than 2 inches long. By exploiting the nemertean talent for copious secretion of slime, land nemerteans manage to live along marine shores, in moist earth, or under foliage and fallen logs, in such places as Bermuda, Australia, New Zealand, and many South Pacific islands. In the Seychelles, Geonemertes arboricola occupies the leaf bases of a screw pine (Pandanus) tree, often living high in the tree.
Only Carcinonemertes has been classed as a parasite. It lives on the gills of various crabs when it is young, and then moves to the egg masses, both feeding on the eggs and living as a commensal by eating any small animals it can find as it clings to its host. Adults of Carcinonemertes carcinophila are about 1 inch long and orange- or brick-red.
Commensal nemerteans live mostly in tunicates, sponges, or bivalves, sharing the food in the host's feeding currents. Common in the mantle cavity of various clams on European and both American coasts is Malacobdella grossa, a short, white, thick worm, with an adhesive disk at the rear. It creeps in leechlike fashion. The genus to which it belongs constitutes a separate order of nemerteans.
The other three orders contain all the more typical elongated worms; they are distinguished from each other mostly by internal characters, such as the arrangement of the muscle layers. The paleonemerteans, with an unarmed proboscis, include such forms as Tubulanus. Also with unarmed proboscis are the heteronemerteans, among them Lineus and Cerebratulus. The latter is a very large, firm, and flattened worm which lives in burrows in sand or mud and swims actively through the water. The hoplonemerteans, with an armed proboscis, are divided into two suborders. In one the members have at the tip of the proboscis a single stylet, a straight or curved thorn which pierces and holds prey. These include many quite common shore forms such as Amphiporus; the very slender Emplectonema, found among mussels and barnacles on pilings; and Paranemertes peregrina of the American west coast, often a rich purple on the upper surface. The parasitic or commensal Carcinonemertes belongs here, as do various commensal species, the fresh-water forms, and also the land nemerteans. In the second suborder, members have on the tip of the proboscis not one large stylet but a large number of minute ones. These worms include some shore species, but most float or swim in the open sea far below the surface. Many are broad, flattened worms, of yellow, orange, pink, or red hues. The drifting types are quite gelatinous, the swimming ones equipped with tail and sometimes also with side fins.
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