The Haplotaxida are one of two 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...

s within the annelid

Annelid

The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...

 subclass

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 Oligochaeta

Oligochaeta

Oligochaeta is a subclass of animals in the biological phylum Annelida, which is made up of many types of aquatic and terrestrial worms, and this includes all of the various earthworms...

, the other being the Lumbriculida. No real common name exists, but they are simply referred to as haplotaxids.

Given that the other clitellata

Clitellata

Clitellata is a class of Annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycle. The clitellates comprise around 8,000 species...

n annelids are embedded between and around the Haplotaxida and Lumbriculida, the traditional Oligochaeta are a paraphyletic assemblege. Thus, the Haplotaxida might eventually be up-ranked to subclass

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 status within the Clitellata or an expanded Oligochaeta, with the present suborders advancing to order rank. The latter – though without merging the Oligochaeta and Clitellata – has been proposed times and again in the past, most prominently for the distinct Moniligastrina.

Families

Of the four suborders of Haplotaxida, two are minor lineages, monotypic

Monotypic

In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...

 at family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 level. Another one, the Tubificina, is sizeable and contains the aquaworms, while the fourth, the earthworm

Earthworm

Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...

s or Lumbricina, unites the bulk of the order's families:

Suborder Haplotaxina

• Haplotaxidae

Suborder Moniligastrina

• Moniligastridae

Suborder Lumbricina

• Alluroididae
• Eudrilidae
  Eudrilidae
  Eudrilidae is a family of earthworms, mostly of Africa. One species, Eudrilus eugeniae , is widely distributed around the warmer parts of the world and cultured as the "African nightcrawler"....
  
  
• Glossoscolecidae
  Glossoscolecidae
  Glossoscolecidae is a large family of earthworms which has native representatives in South and Central America. The species Pontoscolex corethrurus has a circumtropical distribution....
  
  
• Lumbricidae
  Lumbricidae
  The Lumbricidae is a family of earthworms which includes most of the earthworm species well known to Europeans. About 33 lumbricid species have become naturalized around the world, but the bulk of the species are in Holarctic: from Canada and the United States The Lumbricidae is a family of...
  
  
• Hormogastridae
• Ailoscolidae
• Lutodrilidae
• Sparganophilidae
• Criodrilidae
  Criodrilidae
  The family Criodrilidae is represented by genera Criodrilus and Biwadrilus that are limicolous and/or aquatic earthworms endemic to the Palaearctic currently known only from Europe and Japan, respectively...
  
  
• Ocnerodrilidae
• Acanthodrilidae
  Acanthodrilidae
  Acanthodrilidae is an ancient and widely distributed family of earthworms which has native representatives in Australia, New Zealand, South Africa, South America and North America. Interestingly, no native species are known from India nor Asia...
  
  
• Octochaetidae
• Exxidae
• Megascolecidae
  Megascolecidae
  Megascolecidae is a large family of earthworms which has native representatives in Australia, New Zealand, Southeast and East Asia, and North America. The most ancient lineages of the family show a Gondwanan distribution and have been used as evidence of continental drift. Members of the Pheretima...
  
  
• Microchaetidae

Suborder Tubificina

• Dorydrilidae
• Enchytraeidae
  Enchytraeidae
  The Enchytraeidae are a microdrile oligochaete family. They resemble small earthworms and include both terrestrial species known as potworms that live in highly organic terrestrial environments, as well as some that are marine...
  
  
• Naididae (including Tubificidae)
• Opistocystidae
• Phreodrilidae