# Cephalaspidea
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{{Short description|Order of gastropods}}
{{About|the gastropod taxon|the prehistoric fish taxon|Cephalaspida}}
{{Use dmy dates|date=October 2023}}
{{Automatic taxobox
| image = Chelidonura varians.jpg
| image_caption = A live individual of ''[Chelidonura varians](/source/Chelidonura_varians)'' in the family [Aglajidae](/source/Aglajidae), head end towards the lower left
| display_parents = 3
| taxon = Cephalaspidea
| subdivision_ranks = Superfamilies
| subdivision = See text
}}
[https://nudibranchdomain.org/product-category/cephalaspidea-order/colpodaspididae/ The] order '''Cephalaspidea''', also known as the ''' headshield slugs''' and '''bubble snails''', is a major [taxon](/source/taxon) of sea slugs and bubble snails, [marine](/source/marine_(ocean)) [gastropod](/source/gastropod) [mollusk](/source/mollusk)s within the larger [clade](/source/clade) [Euopisthobranchia](/source/Euopisthobranchia).[{{Cite journal |last1=Jörger |first1=Katharina M |last2=Stöger |first2=Isabella |last3=Kano |first3=Yasunori |last4=Fukuda |first4=Hiroshi |last5=Knebelsberger |first5=Thomas |last6=Schrödl |first6=Michael |date=2010 |title=On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia |journal=BMC Evolutionary Biology |language=en |volume=10 |issue=1 |pages=323 |doi=10.1186/1471-2148-10-323 |doi-access=free |pmc=3087543 |pmid=20973994|bibcode=2010BMCEE..10..323J }}] '''Bubble shells''' is another common name for these families of marine gastropods, some of which have thin bubble-like shells.[{{Cite book |last=Dance |first=S. P. |title=Shells. The visual guide to over 500 species of seashell from around the world |publisher=[Dorling Kindersley](/source/Dorling_Kindersley) |year=1996 |isbn=0-86318-811-7 |pages=200–203}}] This clade contains more than 600 species.[{{cite journal | url=https://link.springer.com/article/10.1007%2Fs13127-013-0151-5 | doi=10.1007/s13127-013-0151-5 | title=Flashback and foreshadowing—a review of the taxon Opisthobranchia | date=2014 | last1=Wägele | first1=Heike | last2=Klussmann-Kolb | first2=Annette | last3=Verbeek | first3=Eva | last4=Schrödl | first4=Michael | journal=Organisms Diversity & Evolution | volume=14 | pages=133–149 | doi-access=free }}]
Headshield slugs are the most morphologically diverse group of all the opisthobranchs.{{Citation needed|date=December 2010}}
==Anatomy==
The vast majority of headshield slugs 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 wing-like flaps).
===Life habits and related anatomical structures===
Headshield slugs often live just beneath the surface of the sand and can also be seen crawling on rocks. They have well-developed [sensory structures](/source/Sensory_organs_of_gastropods) to detect prey, which may be other opisthobranchs, [polychaetes](/source/Polychaeta) or bristleworms and [foraminifera](/source/foraminifera)ns. Several species are voracious carnivores.
Members of the brightly colored genus ''[Chelidonura](/source/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](/source/Hancock's_organ) is a [chemosensory organ](/source/Sensory_organs_of_gastropods) 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](/source/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](/source/radula), gizzard, penis, and Hancock's organ.
In 2015 a new study based on [molecular phylogenetics](/source/molecular_phylogenetics) has changed significantly the taxonomy of the Cephalaspidea.[{{cite journal | url=https://dx.doi.org/10.1016/j.ympev.2015.04.011 | doi=10.1016/j.ympev.2015.04.011 | title=A new phylogeny of the Cephalaspidea (Gastropoda: Heterobranchia) based on expanded taxon sampling and gene markers | date=2015 | last1=Oskars | first1=Trond R. | last2=Bouchet | first2=Philippe | last3=Malaquias | first3=Manuel António E. | journal=Molecular Phylogenetics and Evolution | volume=89 | pages=130–150 | pmid=25916189 | bibcode=2015MolPE..89..130O | hdl=1956/10087 | hdl-access=free }}] The [monophyly](/source/monophyly) of the Cephalaspidea was confirmed, but the families Cylichnidae, Diaphanidae, Haminoeidae, Philinidae, and Retusidae were found non-monophyletic. This had led to the creation of new families (([Alacuppidae](/source/Alacuppidae), [Colinatydidae](/source/Colinatydidae), [Colpodaspididae](/source/Colpodaspididae), [Mnestiidae](/source/Mnestiidae), [Philinorbidae](/source/Philinorbidae)) ) and one new genus (''[Alacuppa](/source/Alacuppa)''). Two family names ([Acteocinidae](/source/Acteocinidae), [Laonidae](/source/Laonidae)) and two genera (''[Laona](/source/Laona_(gastropod))'', ''[Philinorbis](/source/Philinorbis)'') are reinstated as valid
=== Linnaean taxonomy ===
* Suborder Cephalaspidea P. Fischer, 1883
** Superfamily [Acteonoidea](/source/Acteonoidea) [D'Orbigny](/source/Alcide_d'Orbigny), 1835
** Superfamily [Bulloidea](/source/Bulloidea) [Lamarck](/source/Jean-Baptiste_Lamarck), 1801
** Superfamily [Cylindrobulloidea](/source/Cylindrobulloidea) [Thiele](/source/Johannes_Thiele_(zoologist)), 1931 – These are now included in the suborder [Sacoglossa](/source/Sacoglossa)
** Superfamily [Diaphanoidea](/source/Diaphanoidea) Odhner, 1914
** Superfamily [Haminoeoidea](/source/Haminoeoidea) Pilsbry, 1895
** Superfamily [Philinoidea](/source/Philinoidea) [J.E. Gray](/source/John_Edward_Gray), 1850
** Superfamily [Ringiculoidea](/source/Ringiculoidea) Philippi, 1853
=== 2005 taxonomy ===
In the [taxonomy of Bouchet & Rocroi](/source/Taxonomy_of_the_Gastropoda_(Bouchet_%26_Rocroi%2C_2005)) (2005), the [clade](/source/clade) Cephalaspidea is arranged as follows:
* Superfamily [Bulloidea](/source/Bulloidea): family [Bullidae](/source/Bullidae)
* Superfamily [Diaphanoidea](/source/Diaphanoidea): families [Diaphanidae](/source/Diaphanidae) and [Notodiaphanidae](/source/Notodiaphanidae),
* Superfamily [Haminoeoidea](/source/Haminoeoidea): families [Haminoeidae](/source/Haminoeidae), [Bullactidae](/source/Bullactidae) and [Smaragdinellidae](/source/Smaragdinellidae)
* Superfamily [Philinoidea](/source/Philinoidea): families [Philinidae](/source/Philinidae), [Aglajidae](/source/Aglajidae), [Cylichnidae](/source/Cylichnidae), [Gastropteridae](/source/Gastropteridae), [Philinoglossidae](/source/Philinoglossidae), [Plusculidae](/source/Plusculidae) and [Retusidae](/source/Retusidae)
* Superfamily [Runcinoidea](/source/Runcinoidea): families [Runcinidae](/source/Runcinidae) and [Ilbiidae](/source/Ilbiidae)
The superfamily [Acteonoidea](/source/Acteonoidea) has been included into the new Informal Group "Lower Heterobranchia" and the superfamily Cylindrobulloidea becomes part of the Group Cylindrobullida.
=== 2009 taxonomy ===
Malaquias et al. (2009)[{{Cite journal |last1=Malaquias |first1=Manuel António E |last2=Mackenzie-Dodds |first2=Jacqueline |last3=Bouchet |first3=Philippe |last4=Gosliner |first4=Terrence |last5=Reid |first5=David G |date=2009 |title=A molecular phylogeny of the Cephalaspidea sensu lato (Gastropoda: Euthyneura): Architectibranchia redefined and Runcinacea reinstated |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1463-6409.2008.00354.x |journal=Zoologica Scripta |language=en |volume=38 |issue=1 |pages=23–41 |doi=10.1111/j.1463-6409.2008.00354.x |url-access=subscription }}] have rearranged taxonomy of Cephalaspidea ''sensu lato'':
* reinstated [Architectibranchia](/source/Architectibranchia)[Malaquias M. A. E. (2010). "Systematics, phylogeny, and natural history of ''Bullacta exarata'' (Philippi, 1849): an endemic cephalaspidean gastropod from the China Sea". ''[Journal of Natural History](/source/Journal_of_Natural_History)'' '''44'''(33 & 34): 2015-2029. {{doi|10.1080/00222933.2010.487574}}.] moved ''[Bullacta exarata](/source/Bullacta_exarata)'' (formerly the only member of Bullactidae) into the family [Haminoeidae](/source/Haminoeidae).