Cephalaspidea

Headshield slugs
The sea slug Chelidonura varians
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
(unranked):	clade Heterobranchia informal group Opisthobranchia clade Cephalaspidea
Superfamilies
See text

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

Members of this worldwide clade appear to be the most primitive of the opisthobranchs.

Headshield slugs are the most morphologically diverse group of all opisthobranchs.

Anatomy

The vast majority possess a shell, although it may be reduced or internal. They have a well-developed headshield, a characteristic broadening at the head, which is used to plow beneath the surface of the sand. This headshield prevents the sand entering the mantle cavity. There is a muscular foot with or without parapodia (= fleshy winglike flaps).

Life habits and related anatomical structures

Headshield slugs live just beneath the surface of the sand and can also be seen crawling on rocks. They have well-developed sensory structures to detect prey, which may be other opisthobranchs, polychaetes or bristleworms and foraminiferans. Several species are voracious carnivores.

Members of the brightly colored genus Chelidonura also have well-developed eyes on the anterior end of the head and bundles of sensory cilia around the mouth. With these cilia they are able to track their prey by following the victim’s mucous trail.

The Hancock's organ is a chemosensory organ situated between the foot and the headshield. It plays a role in olfactory and sensory detection. It is visible as a dark brown pit at the base of the right rhinophore.

Taxonomy

The taxonomy of the shelled cephalaspideans, the bubble snails, like that of many shelled mollusks, used to be based very simply on shell characteristics. But because there are some similarities in shell morphology throughout this group, more recently taxonomists have taken other anatomical characteristics into consideration, such as the radula, gizzard, penis, and Hancock’s organ.

• Suborder Cephalaspidea P. Fischer, 1883
  • Superfamily Acteonoidea D'Orbigny, 1835
  • Superfamily Bulloidea Lamarck, 1801
  • Superfamily Cylindrobulloidea Thiele, 1931 (These are now included in the suborder Sacoglossa)
  • Superfamily Diaphanoidea Odhner, 1914
  • Superfamily Haminoeoidea Pilsbry, 1895
  • Superfamily Philinoidea J.E. Gray, 1850
  • Superfamily Ringiculoidea Philippi, 1853

In the new taxonomy of Bouchet & Rocroi (2005), the clade Cephalaspidea is arranged as follows :

• Superfamily Bulloidea : family Bullidae
• Superfamily Diaphanoidea : families Diaphanidae and Notodiaphanidae,
• Superfamily Haminoeoidea : families Haminoeidae, Bullactidae and Smaragdinellidae
• Superfamily Philinoidea : families Philinidae, Aglajidae, Cylichnidae, Gastropteridae, Philinoglossidae, Plusculidae and Retusidae
• Superfamily Runcinoidea : families Runcinidae and Ilbiidae

The superfamily Acteonoidea has been included into the new Informal Group "Lower Heterobranchia" and the superfamily Cylindrobulloidea becomes part of the Group Cylindrobullida.

References

Wikispecies has information related to: Cephalaspidea