Editing Invertebrate (section)

== Taxonomic significance ==

The term ''invertebrates'' does not describe a [[taxon]] in the same way that [[Arthropod]]a, [[Vertebrata]] or [[Manidae]] do. Each of those terms describes a valid taxon,  [[phylum]], [[subphylum]] or [[Family (biology)|family]]. "Invertebrata" is a term of convenience, not a taxon; it has very little [[Circumscription (taxonomy)|circumscriptional]] significance except within the [[Chordate|Chordata]]. The Vertebrata as a [[Taxonomic rank|subphylum]] comprises such a small proportion of the [[Animal|Metazoa]] that to speak of the [[Kingdom (biology)|kingdom]] [[Animal]]ia in terms of "Vertebrata" and "Invertebrata" has limited practicality. In the more formal taxonomy of Animalia other attributes that logically should precede the presence or absence of the vertebral column in constructing a [[cladogram]], for example, the presence of a [[notochord]]. That would at least circumscribe the Chordata. However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry<ref name=" Pech">{{cite book | last = Pechenik | first = Jan | title = Biology of the Invertebrates | publisher = Wm. C. Brown Publishers | location = Dubuque | year = 1996 | isbn = 978-0-697-13712-8 }}</ref> or perhaps [[Body plan|Bauplan]].<ref>{{cite book |last1=Brusca |first1=Richard C. |last2=Brusca |first2=Gary J. | title = Invertebrates |url=https://archive.org/details/invertebrates0000brus  |url-access=registration  | publisher = Sinauer Associates | location = Sunderland | year = 1990 | isbn = 978-0-87893-098-2 }}</ref>

Despite this, the concept of ''invertebrates'' as a taxon of animals has persisted for over a century among the [[laity]],<ref name="isbn0-19-861271-0">{{cite book |author=Brown, Lesley |title=The New shorter Oxford English dictionary on historical principles |publisher=Clarendon |location=Oxford [Eng.] |year=1993 |isbn=978-0-19-861271-1 |url=https://archive.org/details/newshorteroxford00lesl }}</ref> and within the zoological community and in its literature it remains in use as a term of convenience for animals that are not members of the Vertebrata.<ref name="Agassiz2013">{{cite book |first=Louis |last=Agassiz |title=Essay on Classification |url=https://books.google.com/books?id=O6fDAgAAQBAJ&pg=PT115 |date=21 March 2013 |publisher=Courier Corporation |isbn=978-0-486-15135-9 |pages=115–}}</ref> The following text reflects earlier scientific understanding of the term and of those animals which have constituted it. According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external. They include hugely varied [[body plan]]s. Many have fluid-filled, hydrostatic skeletons, like [[jellyfish]] or worms. Others have hard [[exoskeleton]]s, outer shells like those of [[insect]]s and [[crustacean]]s. The most familiar invertebrates include the [[Protozoa]], [[Porifera]], [[Coelenterata]], [[Platyhelminthes]], [[Nematoda]], [[Annelida]], [[Echinodermata]], [[Mollusca]] and [[Arthropoda]]. Arthropoda include [[insect]]s, [[crustacean]]s and [[arachnid]]s.