Lower Heterobranchia, also known as the Allogastropoda, is a group of rather specialized, highly-evolved sea slug

Slug

Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...

s and sea snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, (marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 gastropod mollusks) within the clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Heterobranchia

Heterobranchia

Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....

. Although the great majority of Lower Heterobranchs are indeed marine, a few have succeeded in making the transition to freshwater

Freshwater

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

.

Description

The shell

Gastropod shell

The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

 shapes in this group are typically those that are seen in the sundial, pyramid, rissoella and orbitestellid families of snails.

2005 taxonomy

In the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005

Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)

The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...

, the Lower Heterobranchia is an Informal Group. Superfamilies within the Lower Heterobranchia include:

• Unassigned to a superfamily (orbitestellid-shells)
  • Family Cimidae
    Cimidae
    The Cimidae is a taxonomic family of sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.- Genera :* Cima Chaster, 1898* Graphis Jeffreys, 1867- External links :...
    
    
  • † Family Dolomitellidae
    Dolomitellidae
    The Dolomitellidae is an extinct taxonomic family of fossil sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.- References :*...
    
    
  • † Family Heterosubulitidae
    Heterosubulitidae
    †Heterosubulitidae is an extinct taxonomic family of sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia....
    
    
  • † Family Kuskokwimiidae
    Kuskokwimiidae
    Kuskokwimiidae is an extinct taxonomic family of fossil sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.- References :*...
    
    
  • † Family Misurinellidae
    Misurinellidae
    The Misurinellidae is an extinct taxonomic family of sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.-Genera:Genera within the family Misureinellidae include:* Misurinella Bandel, 1994, the type genus...
    
    
  • Family Orbitestellidae
    Orbitestellidae
    Orbitestellidae are a family of minute sea snails, marine gastropod molluscs or micromolluscs in the informal group Lower Heterobranchia.-Genera:Genera within the family Orbitestellidae include:* Microdiscula* Orbitestella Iredale, 1917...
    
    
  • Family Tjaernoeiidae
    Tjaernoeiidae
    Tjaernoeia is a genus of sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.Tjaernoeia is the only genus in the family Tjaernoeiidae.- Species :Species in the genus Tjaernoeia include:...
    
    
  • Family Xylodisculidae
    Xylodisculidae
    The Xylodisculidae is a taxonomic family of sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.- References :*...
    
    

• superfamily Acteonoidea
  Acteonoidea
  Acteonoidea is a superfamily of sea snails, or bubble snails, marine gastropod mollusks.- Taxonomy :In the taxonomy of Bouchet & Rocroi , the superfamily Acteonoidea has been included into the informal group "Lower Heterobranchia" , also known as the Allogastropoda.Only one of analyses by Jörger...
  
  
• superfamily Architectonicoidea
• superfamily Glacidorboidea
• superfamily Mathildoidea
  Mathildoidea
  Mathildoidea is a superfamily of sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.-Families:Families within the superfamily Mathildoidea include:*Family Mathildidae* † Family Ampezzanildidae* † Family Anoptychiidae...
  
  
• † superfamily Nerineoidea
  Nerineoidea
  †Nerineoidea is a superfamily of extinct sea snails, fossil marine gastropod mollusks in the informal group Lower Heterobranchia.-Families:Families within the superfamily Nerineoidea include:* † Family Nerineidae* † Family Ceritellidae...
  
  
• superfamily Omalogyroidea
  Omalogyroidea
  Omalogyroidea is a superfamily of minute sea snails, marine gastropod mollusks or micromollusks in the informal group Lower Heterobranchia....
  
  
• superfamily Pyramidelloidea
  Pyramidelloidea
  Pyramidelloidea is a superfamily of mostly very small sea snails, marine gastropod mollusks and micromollusks within the clade Panpulmonata.This is a voluminous taxon: above the species level close to 400 named taxa are referred to this gastropod superfamily.Pyramidelloidea has both fossil and...
  
  
• superfamily Ringiculoidea
• superfamily Rissoelloidea
• † superfamily Streptacidoidea
  Streptacidoidea
  Streptacidoidea is a superfamily of extinct sea snails, fossil marine gastropod mollusks in the informal group Lower Heterobranchia....
  
  
• superfamily Valvatoidea
  Valvatoidea
  Valvatoidea is a superfamily of minute freshwater and marine snails, aquatic gastropod mollusks in the informal group Lower Heterobranchia.-Taxonomy:*Family Valvatidae*Family Cornirostridae*Family Hyalogyrinidae* † Family Provalvatidae...
  
  

(Taxa that are exclusively fossil are indicated with a dagger †)

For a more detailed taxonomy see: Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)#Informal group "Lower Heterobranchia" (= Allogastropoda)

2010 taxonomy

Jörger et al. (2010) have redefined major groups within the Heterobranchia: they moved Glacidorboidea to Panpulmonata

Panpulmonata

Panpulmonata is a taxonomic clade of snails and slugs in the clade Heterobranchia within the clade Euthyneura.Panpulmonata was established as a new taxon by Jörger et al...

 and Pyramidelloidea

Pyramidelloidea

Pyramidelloidea is a superfamily of mostly very small sea snails, marine gastropod mollusks and micromollusks within the clade Panpulmonata.This is a voluminous taxon: above the species level close to 400 named taxa are referred to this gastropod superfamily.Pyramidelloidea has both fossil and...

also to Panpulmonata.