Scalidophora is a group of marine pseudocoelomate

Body cavity

By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...

 invertebrate

Invertebrate

An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s, consisting of the three phyla Kinorhyncha

Kinorhyncha

Kinorhyncha is a phylum of small marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos...

, Priapulida

Priapulida

Priapulida is a phylum of marine worms. They are named for their extensible spiny proboscis, which, in some species, may have a shape like that of a human penis...

, and Loricifera

Loricifera

Loricifera is a phylum of very small to microscopic marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their size ranges from 100 µm to ca....

. The members of the group share a number of characteristics, including introvert larvae and moulting of the cuticle (ecdysis

Ecdysis

Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...

). Their closest relatives are thought to be the Panarthropoda

Panarthropoda

Panarthropoda is a taxon combining the phyla Arthropoda, Tardigrada, and Onychophora. Dunn et al. in 2008 suggested that the tardigrades could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods...

, Nematoda and Nematomorpha

Nematomorpha

Nematomorpha is a phylum of parasitic animals that are superficially morphologically similar to nematode worms, hence the name. They range in size in most species from long and can reach in extreme cases up to 2 metres, and in diameter...

; they are thus placed in the group Ecdysozoa

Ecdysozoa

Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...

.

The two species in the genus Markuelia

Markuelia

Markuelia is a genus of fossil worm-like bilaterian animals allied to Ecdysozoa and known from strata of Lower Cambrian to Lower Ordovician age containing five species....

, known from fossilized embryos from the middle Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

, are thought to be stem Scalidophorans.

The group has also been considered a single group, Cephalorhyncha, with three classes.