Enopla

Not to be confused with Enoplea, a clade of nematodes or roundworms.See text. Not to be confused with Enoplea, a clade of nematodes or roundworms. Enopla is one of the classes of the worm phylum Nemertea, characterized by the presence of a peculiar armature of spines or plates in the proboscis. The record of nemerteans is extremely sparse, as would be expected from a soft-bodied animal. The Cambrian fossil, Amiskwia, has been interpreted as a nemertean based on its resemblance to some pelagic ribbon worms; however, this interpretation is disputed by many paleontologists. The enoplan nemerteans have been regarded as highly derived based on a more complicated muscle arrangement in the body wall and a more complex nervous system. However, whether this is a plesiomorphic or apomorphic character is not clear, and recent molecular studies are inconclusive in this respect. Class Enopla used to be divided in two subclasses, Hoplonemertea and Bdellonemertea, but recent phylogenetic analyses based on nucleotide sequences show that Bdellonemertea should be included in Hoplonemertea. Hoplonemertea (in the old sense) contains two suborders, Monostilifera and Polystilifera. The encompasses those animals with a proboscis armature consisting of a single central stylet on a large cylindrical basis. The Polystilifera are armed with a pad, or shield, bearing numerous small stylets. The Polystilifera are further divided in two taxa, one (Pelagica) containing the pelagic species, and the other (Reptantia) with crawling or burrowing forms. The class is currently divided into 30 families and 155 genera with approximately 650 described species. The two largest genera, Amphiporus and Tetrastemma contain 230 species, i.e., one third of all named species in the class. However, it must be made very clear that the systematics and classification of nemerteans are not based on a phylogenetic approach, and recent studies question the classification. Enoplan nemerteans are generally small, from less than 1 centimetre (0.39 in) up to 10 centimetres (3.9 in), although larger species exist. While most nemerteans are rather drab in color, others are more conspicuous with striking pigment patterns and coloration. However, the more brightly colored forms are more common in the class Anopla. A nemertean is generally recognized in the field by the way it moves. Its normal movement is gliding over the surface by help of cilia on the ventral side in combination with mucus produced by the worm. Some species may, under certain circumstances, swim with undulating movements, but only for a short period of time. Enoplans are differentiated by the proboscis, which is armed (i.e., there is a stylet attached to it) in enoplans but unarmed in anoplans. Enoplans have a common opening for the proboscis and mouth, whereas anoplans have separate openings for the two structures. Enoplan nemerteans are known from all continents and all seas. Terrestrial nemerteans are mainly known from islands in the tropical and subtropical regions, although there are few more widespread species. Freshwater species are also reported from all continents, except the Antarctic. Enoplan nemerteans are typically found in the sea, in the littoral among algae. While larger species may be found simply by turning over boulders, smaller species are not found unless special techniques are utilized. An easy way of collecting nemerteans is to place seaweed and smaller algae in a bucket of sea water and let it stand for a few hours, and up to a couple of days, depending on weather and temperature. The worms will crawl to the sides of the bucket, where they are easily observed and collected, as the oxygen concentration decreases in the water.