Palaeacanthocephala ("ancient thornheads") is a

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

within the

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

Acanthocephala Acanthocephala ( Greek , ' 'thorn' + , ' 'head') is a group of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses t ...

. The adults of these

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

platyzoa The "Platyzoa" are a group of protostome unsegmented animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the phylum Platyhelminthes (or flatworms), and a new phylum, the Acanthognatha, into which he gathered s ...

ns feed mainly on

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

, aquatic

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s and

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s. This order is characterized by the presence of lateral longitudinal lacunar canals and a double-walled proboscis receptacle. The nuclei of the

hypodermis The subcutaneous tissue (), also called the hypodermis, hypoderm (), subcutis, or superficial fascia, is the lowermost layer of the integumentary system in vertebrates. The types of cells found in the layer are fibroblasts, adipose cells, and ...

(outer layer of skin) are fragmented and the males have two to seven cement glands, unlike their relatives the

Archiacanthocephala Archiacanthocephala is a class within the phylum of Acanthocephala. They are parasitic worms that attach themselves to the intestinal wall of terrestrial vertebrates, including humans. They are characterised by the body wall and the lemnisci (whi ...

, which always have eight. There are three

orders Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * H ...

in the class Palaeacanthocephala: *

Echinorhynchida Echinorhynchida is an Order (biology), order of parasitic worms in the phylum Acanthocephala. It contains 13 known families: *Arhythmacanthidae Yamaguti, 1935 *Cavisomidae Meyer, 1932 *Diplosentidae Tubangui and Masiluñgan, 1937 *Echinorhynchida ...

Southwell and Macfie, 1925 * Heteramorphida Amin and Ha, 2008 *

Polymorphida Polymorphida are an order of thorny-headed worms (phylum Acanthocephala). The adults of these parasitic platyzoans feed mainly on fish and aquatic birds. This order contains 5 families:Huston, D. C., Cribb, T. H., & Smales, L. R. (2020). Molecu ...

Petrochenko, 1956

References

Acanthocephalans {{Acanthocephalan-stub