Also found in: Dictionary, Thesaurus, Wikipedia.
A phylum of bilaterally symmetrical, un-segmented, ribbonlike worms, frequently referred to as the Nemertinea. They have an eversible proboscis and a complete digestive tract with an anus. There is no coelom or body cavity, and the mesenchyme or parenchyma and the muscle fibers fill the area between the ciliated epidermis and the cellular lining of the digestive tract. Many species are brightly colored, sometimes having stripes or transverse bars.
The nemertineans are the simplest animals with a circulatory system. There are two lateral blood vessels and in some a third, unpaired dorsal vessel. The blood consists of a colorless fluid which may contain blood cells of several types. In species in which the blood is colored, the pigment is present in the cells. There is no heart, but the walls of the principal vessels may be contractile.
The nervous system has a pair of cerebral ganglia forming the brain as well as two longitudinal nerve cords and many smaller nerves. The ganglia and lateral cords may contain unusually large neurochord cells. In the epidermis there are scattered sensory nerve cells, probably tactile. A few to many simple eyes, or ocelli, may be present in front of the cerebral ganglia. There are no special respiratory organs; respiration occurs through the body surface. Nemertineans are usually either male or female, but a few individuals have both sex organs. Fertilization occurs outside the body in many species but may be internal in certain forms.
The nemertineans are mostly marine, bottom-dwelling worms, found in greatest numbers along the coasts of northern temperate regions. They live under stones, among the tangled masses of plants, in sand, mud, or gravel, and sometimes form mucus-lined tubes. A few are pelagic, fresh-water, or terrestrial. Certain species are commensal with other animals, but none can be regarded as parasitic in a strict sense.
That the Rhynchocoela represent the most highly organized acoelomate animals is indicated by the circulatory system, the presence of an anus, and the specialization of the epidermis. All groups of animals more, complex than the nemertineans have some kind of cavity, a pseudocoele or coelom, between the body wall and the gut, instead of solid mesenchyme. See Coelom
The phylum Rhynchocoela, containing about 550 known species, is divided into two classes, Anopla and Enopla.