Notaspidea

Notaspidea, also known as the sidegill slugs, is an artificial grouping of sea slugs which is now split into two unrelated groups, the Umbraculida and the Pleurobranchomorpha.^[1]

Notaspidea
A live individual of Pleurobranchaea meckelii; the gill (or ctenidium) is visible in this view of the right-hand side of the animal
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Subclass:	Orthogastropoda (older classification) Heterobranchia (new classification)
Order:	Opisthobranchia (older classification)
Suborder:	Notaspidea P. Fischer, 1883

Taxonomic history

Notaspidea, also known as the sidegill slugs, was a suborder which included both sea slugs and sea snails or false limpets, marine opisthobranch gastropod molluscs in the subclass Orthogastropoda. However, in the newer taxonomy of Bouchet & Rocroi (2005), the families Umbraculidae and Tylodinidae belong to the superfamily Umbraculoidea Dall, 1889, part of the clade Umbraculida. Grande et al. (2004) found Umbraculoidea to be a sister clade to the Cephalaspidea (Acteonoidea excluded).^[2]

The families Tylodinidae and Umbraculidae have large limpet-like external shells and a small mantle, while the species in the family Pleurobranchidae have a prominent mantle and an internal shell that becomes reduced or is lost in adults.^[3] Many species produce mantle secretions as a chemical defense against predators ^[4]

Families

• Pleurobranchidae Menke, 1828
• Tylodinidae Gray, 1847
• Umbraculidae Dall, 1889