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The Phylum Chaetognatha is a small group that consists of 70-80 species of exclusively marine, carnivorous, hermaphroditic animals; with the exception of the benthic genus Spadella, the Chaetognatha are holoplanktonic.
Chaetognaths are bilaterally symmetrical enterocoelous animals, with an elongated cylndrical body; they are usually colourless, transparent or slightly opaque. The body [Chaetognatha-plate ] is divided in three parts by internal partitioning: head, trunk and tail. The head is slightly rounded and separated from the trunk by a constricted neck. Each side of the head bears a group of curved grasping hooks and one or two rows of teeth, called the anterior and posterior teeth; the hooks and teeth are made of chitin. A pair of uniquely arranged pigmented eyespots is present.
In some species, the epidermis is thickened in the neck region and forms a collarette, which may extend right down the body in some species.
The trunk bears one or two pairs of lateral fins, usually overlapping the septum between trunk and tail. The fins are thin, transparent and supported by fin rays. The number, shape and position of the fins are used in the identification of the species. A ventral ganglion is located on the antero-ventral surface of the trunk.
The alimentary canal is divided into three parts: the oesophagus, intestine and rectum; running from the mouth to the anus at the ventral side of the trunk, near the septum between trunk and tail. The anterior end of the intestine may be extended into lateral alimentary diverticula. On each side of the posterior part of the intestine are the ovaries. The shape and maximum length of the ovary depends on the species.
The tail narrows gradually and bears at the end a horizontal tail fin, which is also transparent and supported by fin rays. Between the lateral fins and tail fin are the laterally situated seminal vesicles; these form conspicuous bulges of various shapes on the tail segment and rupture to release the spermatophores when mature. Shape and position of the seminal vesicles vary according to the species. Fertilised eggs are released in the seawater and develop into a larva. The life span of chaetognaths may vary from six weeks to two years, depending on the species.
Chaetognaths are active and rapid swimmers (except for Spadella species that adhere to the substrate by means of papilla but can swim short distances).
Chaetognatha have a worldwide distribution and a wide depth range; some species perform diurnal vertical migration. Chaetognaths can make up a considerable part of the macrozooplankton biomass.

[After Pierrot-Bults and Chidgey, 1988]

The key to the chaetognath species of the North Sea starts at Page 3: Chaetognatha. The following chaetognath species are included in this volume:

Family Eukrohniidae
Eukrohnia fowleri
Eukrohnia hamata
Krohnitta subtilis
Family Pterosagittidae
Pterosagitta draco
Family Sagittidae
Sagitta elegans
Sagitta enflata
Sagitta friderici
Sagitta hexaptera
Sagitta lyra
Sagitta maxima
Sagitta minima
Sagitta serratodentata
Sagitta setosa
Sagitta tasmanica
Family Spadellidae
Spadella cephaloptera

Phylum Chaetognatha