Malaquias, Manuel António Encarnação Natural History Collections, Bergen Museum, University of Bergen, Bergen, Norway.
- Habitat and behavior
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An order of marine gastropods assigned to the informal group Opisthobranchia (phylum Mollusca) and often referred to as head-shield sea slugs or bubble shells. The name Cephalaspidea was introduced in 1887 by the French naturalist Paul Henri Fischer. The main defining characteristic for this group of marine slugs is the presence of a large head-shield, which is a feature considered to be an adaptation to their burrowing habits. The systematics of the order Cephalaspidea has been problematic because of the difficulty of establishing a sound morphological definition for the group, where discrepancies are the rule (for example, the shell can be external, internal, or absent; thick or thin; involute or having an exposed spire; and so forth). As a result, the taxonomic history of the Cephalaspidea has been one of a continuous flux of inclusion and exclusion of taxa. In particular, some problematic families (for example, Acteonidae, Akeridae, Aplustridae, Bullinidae, Diaphanidae, Notodiaphanidae, Philinoglossidae, Ringiculidae, Runcinidae, and Cylindrobullidae) have been moved between the cephalaspids and other major groups of opisthobranch gastropods. See also: Gastropoda; Mollusca; Opisthobranchia
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