Philinoidea is a taxonomic superfamily

Taxonomic rank

In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...

 of sea slugs, specifically headshield 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, 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 in the clade Cephalaspidea

Cephalaspidea

The suborder Cephalaspidea, also known as the headshield slugs and bubble snails, is a clade of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia....

.

Anatomy

They are generally small animals, and have no shell or gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s. Unlike some related forms, the visceral mass is not sharply set off from the rest of the body.

Families

Families within the superfamily Philinoidea include (according to 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...

):

• Aglajidae
  Aglajidae
  Aglajidae is a family of often colorful, medium-sized, sea slugs, marine opisthobranch gastropod mollusks. These are not nudibranchs; instead they are headshield slugs, in the clade Cephalaspidea.-Genera:...
  
  
• Cylichnidae
  Cylichnidae
  Cylichnidae, common name the "chalice bubble snails" or "canoe bubble snails" is a family of sea snails or bubble snails, marine gastropod mollusks in the superfamily Philinoidea.- 2005 taxonomy :...
  
   - synonym: Tornatinidae
• Gastropteridae
  Gastropteridae
  Gastropteridae is a family of sea slugs, gastropod mollusks in the superfamily Philinoidea of the clade Cephalaspidea, the headshield slugs and bubble snails....
  
  
• Philinidae
  Philinidae
  Philinidae is a family of medium-sized, sea slugs, marine opisthobranch gastropod mollusks. They are headshield slugs, in the Cephalaspidea.-Genera:Genera within the family Philinidae include:* Johania Monterosato, 1884* Laona A. Adams, 1865...
  
  
• Philinoglossidae
  Philinoglossidae
  Philinoglossidae is a family of marine snails, gastropod mollusks in the superfamily Philinoidea.- Genera :* Philinoglossa Hertling, 1932** Philinoglossa helgolandica Hertling, 1932** Philinoglossa remanei Marcus & Marcus, 1958...
  
  
• Plusculidae
  Plusculidae
  Pluscula cuica is a species of marine snail, gastropod mollusk in the superfamily Philinoidea within the clade Cephalaspidea.Pluscula cuica is the only species in the genus Pluscula. Genus Pluscula is the only genus in the family Plusculidae....
  
  
• Retusidae
  Retusidae
  Retusidae is a family of small sea snails, marine opisthobranch gastropod mollusks. They are headshield slugs, in the clade Cephalaspidea.-Genera:Genera within the family Retusidae include:* Coleophysis* Cylichnina Monterosato, 1884...
  
  - synonym: Volvulidae