CE5
A. Ribbon worm, representative of the phylum Nemertinea
B. Internal anatomy of a ribbon worm
Nemertinea [key], phylum of elongated, often flattened, marine Worms, sometimes called ribbon worms. There are 900 known species, ranging in size from a fraction of an inch to 90 ft (27 m). The most distinctive structure of the carnivorous ribbon worms is a proboscis that can be shot out to capture food. When not in use the proboscis is sheathed in a muscular tube that lies above the digestive tract. When everted, it can coil about the prey, entangling it in a sticky mucus that is sometimes toxic. In many species the proboscis is tipped with a sharp spike, or stylet, which pierces the prey continuously as the irritating secretions are poured on. The proboscis can also be used for defense or for burrowing into sand or mud. Annelid worms form the bulk of the nemertine diet, although mollusks and other marine animals are also eaten. A ciliated epidermis and several other anatomical structures that bear a close resemblance to those of flatworms (see Platyhelminthes) are characteristic of ribbon worms; however, in the latter group they are generally more complex. The nemertine nervous system is similar to those in higher flatworms, but there is a greater concentration of nervous tissue in the brain and nerve cords. Most nemertines have sense organs, such as light-sensitive pigment cups, similar to those of the planarian flatworms. Nearly all species also have deep ciliated grooves in the head region in close association with the brain. The grooves are believed to have a chemoreceptive or endocrine function. Nemertinea is the most primitive phylum that has developed a closed circulatory system, in which the blood does not bathe the tissues, but is confined to vessels with distinct walls. Excretion is accomplished by tubules that are in intimate contact with the circulatory system. The nemertine digestive tract, in a departure from the blind gut of flatworms, has developed into a tube with an opening at both ends. The presence of a second, or anal, opening at the posterior end permits ingestion and egestion to occur simultaneously. Food is pushed through the digestive tract, and blood moves along the circulatory tubes by the undulating movement of the body-wall musculature, located below the epidermis. The larger worms also move about on the ocean floor by means of such muscle contractions. Smaller types glide along on epidermal cilia over trails of secreted mucus. In most species the sexes are separate. The eggs and sperm are usually shed into the water and fertilized externally. Many nemertines, like the planarians, are also capable of reproducing asexually by fragmentation of the body.
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