Notaspidea, also known as the sidegill slugs, was a suborder which included both sea slug

Sea slug

Sea slug is a common name used for several different groups of saltwater snails that either lack a shell or have only an internal shell, in other words this name is used for various lineages of marine gastropod mollusks that are either not conchiferous or appear not to be.The phrase "sea slug" is...

s and sea snails or false limpet

Limpet

Limpet is a common name for a number of different kinds of saltwater and freshwater snails ; it is applied to those snails that have a simple shell which is more or less conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snails.The name limpet is most...

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...

 opisthobranch gastropod molluscs in the 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...

 Orthogastropoda

Orthogastropoda

Orthogastropoda was a major taxonomic groupings of snails and slugs, an extremely large subclass within the huge class Gastropoda according to the older taxonomy of the Gastropoda ....

.

Adult notaspideans are carnivorous

Carnivore

A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

 or feed on dead matter. Their anatomy is characterized by a single, external ctenidium

Ctenidium

Ctenidium may be:* Ctenidium , , a comb-like gill, part of the respiratory system of gastropods and bivalves* Ctenidium, , a row of peg-like spines in some insects...

 (respiratory apparatus) on the right side, rolled rhinophore

Rhinophore

A rhinophore is one of a pair of club-shaped structures which are the most prominent part of the external head anatomy of a group of sea slugs, marine gastropod opisthobranch mollusks in the order Nudibranchia, the nudibranchs, specifically the dorid nudibranchs.- Etymology :The name relates to the...

s, and a flattened shell.

The families Tylodinidae

Tylodinidae

Tylodinidae is a family of sea snails or limpets, marine opisthobranch gastropod mollusks in the superfamily Umbraculoidea.- 2005 taxonomy :...

 and Umbraculidae

Umbraculidae

Umbraculidae are a taxonomic family of unusual sea snails or limpets, marine opistobranch gastropod molluscs in the clade Umbraculida.-Genera:Genera within the family Umbraculidae include:* Umbraculum Schumacher, 1817* Spiricella Rang, 1828...

 show large limpet-like external shells and a small mantle

Mantle (mollusc)

The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

, while the species in the family Pleurobranchidae

Pleurobranchidae

The Pleurobranchidae are a taxonomic family of sea slugs, marine gastropod molluscs in the order Notaspidea.The species in the family Pleurobranchidae have a prominent mantle and an internal shell that becomes reduced or is lost in adults. Some adult species have been seen feeding on ascidians...

 have a prominent mantle and an internal shell that becomes reduced or is lost in adults . Many species produce mantle secretions as a chemical defense against predators

Families

• Pleurobranchidae
  Pleurobranchidae
  The Pleurobranchidae are a taxonomic family of sea slugs, marine gastropod molluscs in the order Notaspidea.The species in the family Pleurobranchidae have a prominent mantle and an internal shell that becomes reduced or is lost in adults. Some adult species have been seen feeding on ascidians...
  
   Menke, 1828
• Tylodinidae
  Tylodinidae
  Tylodinidae is a family of sea snails or limpets, marine opisthobranch gastropod mollusks in the superfamily Umbraculoidea.- 2005 taxonomy :...
  
   Gray, 1847
• Umbraculidae
  Umbraculidae
  Umbraculidae are a taxonomic family of unusual sea snails or limpets, marine opistobranch gastropod molluscs in the clade Umbraculida.-Genera:Genera within the family Umbraculidae include:* Umbraculum Schumacher, 1817* Spiricella Rang, 1828...
  
   Dall, 1889

However, in the newer taxonomy of Bouchet & Rocroi

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...

 (2005), the families Umbraculidae and Tylodinidae belong to the superfamily Umbraculoidea Dall, 1889, part of 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...

 Umbraculida. Grande et al. (2004) found Umbraculoidea to be a sister clade to the 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....

(Acteonoidea excluded).