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Cephalaspidea The suborder Cephalaspidea or the headshield slugs is a suborder of mollusks from the order Opisthobranchia. Members of this worldwide suborder are considered the most primitive of the Opisthobranchia. They most closely resemble their prosobranch ancestors. Cephalispideans are therefore traditionally considered to be a transitional group between prosobranchs with shell and the shell-less opisthobranchs. Headshield slugs are the most morphologically diverse of all opistobranchs. 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). 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, such as other opisthobranchs, polychaetes or bristleworms and foraminiferans. Several species are voracious carnivores. In addition, members of the brightly colored genus Chelidonura 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 victims mucous trail. The Hancocks 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 headshield slugs relies on differences in their shell. But because of their morphological similarities, other anatomical characteristics have been taken into account, such as the radula, gizzard, penis and Hancocks 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
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