Gnathifera (Spiralia)

In most gnathiferans, the anus opens on the dorsal surface of the animal. Extinct members of the total group of Chaetognatha, such as Timorebestia and Amiskwia known from the Cambrian are larger than living chaetognaths, and retain jaw-like mouthparts which are not found in living chaetognaths as well as lacking the spines present around the mouth of crown-group chaetognaths. They share with chaetognaths the fins running along the sides of the body and at the end of the tail, and the possession of a unique structure called the "ventral ganglion" not found among other animals which controls the movement and senses. These giant stem-chaetognaths are thought to have been free-swimming (nektonic) and occupied a high trophic position than modern chaetognaths. Development All known chaetognathiferans are direct developers, Though chaetognathiferans are included in Spiralia, rotifers and chaetognaths do not exhibit spiral cleavage. Little is known of the development of micrognathozoans. The development of gnathostomulids is poorly known, but they appear to exhibit spiral cleavage. ==Classification==

Chaetognathifera is a member of Spiralia. It is the sister taxon of a clade comprising all other spiralians. Before the cladistic era, most gnathiferans were regarded as aschelminths, a grouping now recognized as polyphyletic. Chaetognaths exhibit numerous morphological similarities to rotifers, suggesting that they may be sister taxa. Rotifera comprises four subclades: Seisonida, Acanthocephala, Bdelloidea, and Monogononta. Acanthocephalans were traditionally excluded from Rotifera, but it is now known that rotifers are paraphyletic without including acanthocephalans. Some taxonomists call the clade of rotifers including acanthocephalans Syndermata, but others continue to use Rotifera and regard acanthocephalans as rotifers. ==Fossil record==

'' a basal gnathiferan from Cambrian of Canada, which is much larger than modern gnathiferans The fossil record of chaetognathiferans is poor. There are no known fossil gnathostomulids. Fossils of the extant rotifer genus Habrotrocha are known from Dominican amber dating to the late Eocene, but rotifers are otherwise only known from the Holocene. By contrast, the chaetognath fossil record, while still patchy, includes numerous Paleozoic specimens. Protoconodonts are stem-group chaetognaths. The earliest protoconodonts date to the Fortunian age of the Cambrian, and are among the oldest known bilaterians. The enigmatic Cambrian taxon Amiskwia is a gnathiferan and has recently been suggested to be a stem-group chaetognath.The Cambrian ectoparasite Inquicus appears to be a gnathiferan. ==History==

Gnathifera was named in 1995 to unite gnathostomulids and rotifers. Micrognathozoans were soon added to this grouping. Chaetognaths, long considered a distinct lineage with no close relatives, were identified as gnathiferans in 2019. and later transformed into the formal name Chaetognathifera, with a proper cladistic definition: "the smallest clade including Adineta vaga (Rotifera) and Parasagitta elegans (Chaethognatha)," while Gnathifera was defined as "the smallest clade including Adineta vaga (Rotifera) and Gnathostomula paradoxa (Gnathostomulida)." ==References==