[4][5] The most distinctive characteristic of gnathiferans is the presence of complex sclerotized mouthparts made of chitin.

[7][8] Unlike other gnathiferans, in chaetognaths and the extinct Amiskwia the anus is located on the ventral surface in a subterminal position.

[11][12][13][14][15][16][17] Extinct members of the total group of Chaetognatha, such as Timorebestia and Amiskwia known from the Cambrian are larger than living chateognaths, 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 callled 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.

[19][2] Gnathostomulida Micrognathozoa Chaetognatha Seisonida Acanthocephala Bdelloidea Monogononta Platytrochozoa Gnathifera is a member of Spiralia.

Chaetognaths exhibit numerous morphological similarities to rotifers, suggesting that they may be sister taxa.

[21][22] The enigmatic phylum Cycliophora may belong to Gnathifera, but other studies suggest that it is more closely related to the Entoprocta.

[27][28] By contrast, the chaetognath fossil record, while still patchy, includes numerous Paleozoic specimens.