Taxonomy
- Class Anopla ("unarmed"). Includes animals with proboscis without stylet, and a mouth underneath and behind the brain.
- Order Paleonemertea. Comprises 100 marine species. Their body wall has outer circular and inner length-wise muscles. In addition, Carinoma tremaphoros has circular and inner length-wise muscles in the epidermis; the extra muscle layers seem to be needed for burrowing by peristalsis.
- Order Heteronemertea. Comprises about 400 species. The majority are marine, but three are freshwater. Their body-wall muscles are disposed in four layers, alternately circular and length-wise starting from the outermost layer. The order includes the strongest swimmers. Two genera have branched proboscises.
- Class Enopla ("armed"). All have stylets except order Bdellonemertea. Their mouth is located underneath and ahead of the brain. Their main nerve cords run inside body-wall muscles.
- Order Bdellonemertea. Includes seven species, of which six live as commensals in the mantle of large clams and one in that of a freshwater snail. The hosts filter feed and all the hosts steal food from them. These nemerteans have short, wide bodies and have no stylets but have a sucking pharynx and a posterior stucker, with which they move like inchworms.
- Order Hoplonemertea. Comprises 650 species. They live in benthic and pelagic sea water, in freshwater and on land. They feed by commensalism and parasitism, and are armed with stylet(s)
- Suborder Monostilifera. Includes 500 species with a single central stylet. Some use the stylet for locomotion as well as for capturing prey.
- Suborder Polystilifera. Includes about 100 pelagic and 50 benthic species. Their pads bear many tiny stylets.
Read more about this topic: Nemertea