Heteronemertea is a monophyletic

Monophyly

In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of about 500 species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of nemertean worm. It contains genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 such as Lineus

Lineus

Lineus is a genus of nemertine worms, including the bootlace worm, arguably the longest animal alive....

and Cerebratulus, and includes the largest and most muscular nemerteans.

Almost all heteronemerteans have three primary body-wall muscle strata: an outer longitudinal, a middle circular, and an inner longitudinal. The lateral nerve cords are outside the circular muscle, as in palaeonemertea

Palaeonemertea

Palaeonemertea is an order of primitive nemertean worm. It may be para- or polyphyletic, consisting of three to five clades and totalling about 100 species....

ns, but separated from the epidermis

Squamous epithelium

In anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...

 by the usually well-developed outer longitudinal muscle.

Taxomomy

Families within the order Heteronemertea include:

• Baseodiscidae
• Cerebratulidae
• Gorgonorhynchidae
• Lineidae
  Lineidae
  Lineidae is a family of nemertean worms. It contains the following genera:* Aetheolineus Senz, 1993* Ammolineus Senz, 2001* Antarctolineus Muller & Scripcariu, 1964* Apatronemertes Wilfert & Gibson, 1974* Australineus Gibson, 1990...
  
  
• Mixolineidae
• Panorhynchidae
• Poliopsiidae
• Polybrachiorhynchidae
• Pussylineidae
• Riseriellidae
• Valenciniidae