Chaetognatha
Morphology

Chaetognaths are commonly called arrow-worms owing to their shape and high swimming velocity [Sagitta in ventral view ]. Indeed, they are elongate and more or less circular in section. Moreover, one or two pairs of lateral fins and a tail fin, all more or less provided with rays, play the role of feathers on arrows. Characters used for their determination are indicated in the figures Sagitta in ventral view and head, dorsal view.
The body is transparent to semi-opaque, sometimes with pigmented parts (often orange or red). It consists of three regions: the head, the trunk, and the tail.
The head, rounded and slightly flattened, bears on each side one set of grasping spines or hooks, a vestibular organ, and one or two (anterior and posterior) rows of teeth. The mouth opens ventrally. There are two dorsal eyes, in most species a central pigment cell surrounded by photoreceptive cells, or in a few species, ommatidia-like structures [different aspects of the eye ]. Eyes are lacking in some deep-living species.
The trunk, separated from the head by a septum, begins as a narrow neck. It contains the gut, sometimes provided with a pair of anterior intestinal diverticula, and the paired female gonads or ovaries. The anus and gonads open just before the trunk-tail septum.
The tail contains the testes and bears laterally a pair of external seminal vesicles. In some species the epidermis is more or less thickened, forming an alveolar tissue named the collarette, which occupies at least the neck area. A large ventral nervous ganglion is easily visible on the anterior part of the trunk, under the epidermis. A dorsal corona ciliata, beginning at the level of the eyes, may extend to the anterior trunk. Many sensitive receptors are scattered on the whole body. A few primitive genera (Archeterokrohnia, Heterokrohnia, Eukrohnia and Bathyspadella) are provided with a pair of so-called "glandular canals" embedded in the collarette tissue, opening posteriorly on the dorso-lateral sides of neck. Their role is unknown, but if Tokioka's observations (1939) are correct in that "[they] seem to terminate anteriorly into the cephalic coelom", they would be excretory coelomoducts. The presence of cilia in the lumen of the canals (Casanova and Duvert, unpublished) might support this view.

Successive, morphologically more or less distinct stages of development are called maturity stages, their number varying from 3 to 6, according to the author. The former is the easiest for common use, since a simplified scale can be applied to all species. The three following stages, based on genital development, are defined according to Furnestin (1957),
— I: ovaries and seminal vesicles not developed or barely visible;
— II: genitals moderately developed, ovaries slender with small ova; seminal vesicles empty;
— III: ovaries and vesicles fully mature, the former swollen, with conspicuous ova; the latter filled with sperm or partly discharged.