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The chaetognaths are small marine invertebrates (from 2 to 120 millimeters in length), mainly planktonic. They are called “arrow worms” because of their transparent body which is long and lean, with lateral and caudal fins. These are predatory swimmers; they have chitinous spines around their mouth -their Greek name means “bristle-jaw”- which can be retracted under a hood. They eat small invertebrates such as copepods, and even alevins. The anatomy of the chaetognaths is very simple: two transverse septa close off the main cavity and delimit a head, a trunk (including the female genital coelom) and a tail (with the male genital coelom). The organisms are hermaphroditic and fertilization is internal. Development is direct. Chaetognaths are found in coastal waters of the Metiterranean sea as well as the Atlantic ocean and the English Channel.
The Chaetognaths constitute a small phylum of about one hundred species. In spite of its modest size, this group has aroused the curiosity of scientists for over a century, because it has not been easy to classify it on the tree of bilaterian animals (animals with bilateral symmetry, i.e. most animals but sponges and jellyfish). In 1844, Darwin described these animals as “remarkable for obscurity of their affinities”. Since then, they have been placed in a large number of different positions. Before the advent of molecular phylogenies, zoologists tended to place them with the Echinoderms (starfish), Hemichordates and Chordates (ascidians, Vertebrates) within the Deuterostome group, i.e. the animals in which the mouth forms secondarily, and is never derived from the first orifice of the embryo. Indeed, the chaetognaths do present this characteristic, as well as another Deuterostome trait: the enterocoelous development of the coelom (i.e., the coelom forms from outpocketings of the embryonic gut). Their photoreceptor cells also bear resemblance with those of the Chordates. However, other characteristics are shared with the Protostomes, the other large group of animals with bilateral symmetry (Mollusks, Annelids, Arthropods, Nematodes, etc.). These include the ventral position of the nervous chain and the presence of chitin. Finally, molecular phylogeny studies, which are sometimes divergent, agree on at least one point: they exclude the chaetognaths from the deuterostomes sensu stricto. However the reliability of these studies is limited by the artefact of long-branch attraction: the rapid evolution of gene sequences studied to date in the chaetognaths may have led to placement of this lineage toward a basal position in the tree.