CHAETOGNATHA, a small group of transparent and for the most part pelagic organisms, whose position is very isolated. There are eight genera and 38 species; the best-known genus is Sagitta with 27 species. These animals exist in extraordinary quantities, so that under certain conditions the surface of the sea seems almost stiff with the incredible multitude of organisms which pervade it. Rough seas cause them to drop into deeper water. Deep-sea forms also occur, but the group is essentially pelagic. The Chaetognatha form part of the diet of pelagic fishes. They are very voracious readily devouring herring fry as big as themselves.
As a rule the body is i to 3 cm. in length, though some species are larger, by 4 or 5 mm. in breadth, and is shaped something like a torpedo with side flanges and a slightly swollen, rounded head. It can be divided into three regions—(i.) head, (ii.) trunk, and (iii.) tail, separated from one another by two transverse septa. The almost spherical head is covered by a hood which can be retracted ; it bears upon its side a number of sickle-shaped, chitinous hooks and rows of low i spines. A pair of eyes lie dorsally.
The trunk contains a spacious body-cavity filled during the breeding season by the swollen ovaries, and the same is true of the tail, substituting testes for ovaries.
The skin consists of a trans parent cuticle excreted by the un derlying ectoderm ; beneath this is a basement membrane ; and then a layer of longitudinal mus cle fibres which are limited in side by a layer of peritoneal cells.
The muscles are striated and ar ranged in four quadrants. Along each side of the body stretches a horizontal fin and a similar flange surrounds the tail.
The mouth opens on to the ventral surface of the head. It leads into a straight alimentary canal whose walls consist of a layer of ciliated cells, ensheathed in a thin layer of peritoneal cells.
There is no armature, and there are no glands, and the whole tract can be divided only into an oesophagus and an intestine. A median mesentery running dorso ventrally supports the alimentary canal and is continued behind it into the tail, thus dividing the body cavity into two lateral halves.
There are no specialized circulatory, respiratory or excretory organs. The nervous system consists of a cerebral ganglion in the head, and a ventral ganglion in the trunk, with lateral commis sures uniting these ganglia on each side. There is a minute but extensive nervous plexus all over the body.
Chaetognatha are hermaphrodite. The ovaries are attached to the side walls of the trunk region ; between them and the body wall lie the two oviducts whose inner and anterior end is closed, their outer ends opening one on each side of the anus, where the trunk joins the tail. This oviduct fits closely over a second duct, the receptaculum seminis, which the spermatozoa enter, passing through the walls and finally reaching the ripe ovum. Temporary oviducts are formed at each oviposition. A number of ova ripen simultaneously. The two testes lie in the tail and the spermatozoa pass out through short vasa deferentia with internal ciliated funnels.
With hardly an exception the transparent eggs are laid into the sea and float on its surface. The development is direct and there is no larval stage. As in some insects, the cells destined to form the reproductive organs are very early apparent.
The great bulk of the group is pelagic, as the transparent nature of all their tissues indicates. They move by flexing their bodies. The Chaetognatha appear to have no close relatives, and probably arose from the primitive Procoelomate stock in very ancient times. Recently Meek has attempted to show that they are related to the vertebrates. (See also PLANKTON.) See G. T. Burfield, "Sagitta" (L.M.B.C. Memoirs, xxviii. 1927) ; Meek, Proz. Zool. Soc. (Lond. 1928) .