This class of Vermes contains two genera, Sagitta and Spadella; pelagic with the exception of Sp. cephaloptera, which is littoral. They occur in all seas. The organism is divided by two septa into three regions, a head, body, and tail. The head has a slit-like mouth on its ventral aspect, is guarded by a hood-like integumental fold on each side, and bears one or two rows of stout spines, and a single row of sickle-shaped setae, or jaws. The body and the tail carry one (Spadella) or two (Sagitta) pairs of lateral fins, and the tail always terminates with a caudal fin. The body is covered by a many-layered squamous ectoderm with an underlying delicate supporting lamina. The fins contain a gelatinous skeletal lamella, on each surface of which lie fine rods extending outwards from the body beneath the ectoderm. The muscles are striated, broken up into special bundles in the head, but in the body and trunk arranged in four bands, two dorsal and two ventral.

The nervous system consists of a supra-oesophageal ganglion in the head, connected by long commissures to an infra-oesophageal ganglion which lies near the centre of the body. They give off various nerves; the. latter especially a very large number, which eventually break up into a fine plexus with nodal ganglion cells. These parts lie in the ectoderm. But two pairs of small ganglia are connected with the supra-oesophageal ganglion, and lie in the mesoderm supplying the muscles of the head. The organs of special sense are: a pair of eyes lying in the ectoderm connected with nerves arising from the supra-oesophageal ganglion; a band of ciliated epithelium behind the eyes which.is disposed in a ring varying in shape in different species, and perhaps olfactory in function;. a variable number of tactile organs scattered over the body, tail, and fins, formed of a central row of cells bearing sense-hairs, surrounded by supporting cells.

The alimentary canal consists of a stomodaeum, or pharynx, lined by a single layer of epithelium, and provided with a thin layer of striated muscle fibres running dorso-ventrally, followed by an intestine (archenteron) lined by a single layer of ciliated epithelial cells, some of which appear to be glandular. The anus is ventral, and placed where the body and tail join. The intestine is suspended by dorsal and ventral mesenteric bands which are fenestrated, and divide the coelome of the body into a right and left half. The coelome is roomy, and lined by a layer of epithelium not visible in all places. The tail also contains a coelome, similarly divided by a vertical partition, which appears to originate in a backward extension of the archenteron at the period when its original cavity is obliterated, and before it has acquired its permanent cavity (Hertwig).

The Chaetognatha are hermaphrodite. There are two ovaries lying one in each half of the coelome of the body. Each ovary has a duct in which sperm is often found. It opens laterally on the dorsal side of the lateral fin, where the body and tail join. Two testes, solid cellular ridges, lie, one in each half of the coelome in the tail on its outer wall. Masses of cells are set free into the coelome, and there differentiate into spermatozoa. Th\e cell masses are in constant motion owing to the presence of cilia on the coelomic epithelium. The spermatozoa are filamentous, transversely striated in Spadella cephaloptera. They escape on each side by a narrow ciliated canal, which has an internal ciliated funnel, and ends in a dilated vesicula seminalis. The latter has an external aperture in the posterior third of the tail.

The ova are enclosed in a membrane, are transparent, and float on the surface of the water, except in the case of Sp. cephaloptera, which attaches them by a gelatinous peduncle to algae. Segmentation is total. There is an invaginate gastrula, and the coelome is formed as a right and left enterocoele. The permanent mouth is formed at the opposite pole to the gastropore, which is closed. There is a stomodaeum; the mode of formation of the anus is unknown. The sexual glands are derived from two cells of the archenteron, which differentiate early, and divide each into two, an ovarian and testicular cell or rudiment.

Spadella Marioni, Gourret, An. Mus. Nat. Hist. Marseille, ii. 1884-5.

Die Chaetognathen, O. Hertwig, J. Z. xiv. 1880; cf. Die Wurmfauna von Madeira, Langerhans, Z. W. Z. xxxiv. 1880. I Chaetognathi, Grassi, Monograph v, Fauna und Flora des Golfes von Neapel, 1883.