The nervous system consists of a cerebral ganglion in the head, a conspicuous ventral ganglion in the trunk, and of lateral commissures uniting these ganglia on each side. The whole of this system has retained its primitive connexion with the ectoderm. The cerebral ganglion also gives off a nerve on each side to a pair of small-ganglia, united by a median commissure, which have sunk into and control the muscles of the head. As in other animals there is a minute but extensive nervous plexus, which permeates the whole body and takes its origin from the chief ganglia. In addition to the eyes and the olfactory circle on the head scattered tactile papillae are found on the ectoderm.
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 described as closed, their outer ends opening one on each side of the anus, where the trunk joins the tail. According to Miss N.M. Stevens the so-called oviduct acts only as a “sperm-duct” or receptaculum seminis. The spermatozoa enter it and pass through its walls and traverse a minute duct formed of two accessory cells, and finally enter the ripe ovum. Temporary oviducts are formed between the “sperm-duct” and the germinal epithelium at each oviposition. A number of ova ripen simultaneously. The two testes lie in the tail and are formed by lateral proliferations of the living peritoneal cells. These break off and, lying in the coelomic fluid, break up into spermatozoa. They pass out through short vasa deferentia with internal ciliated funnels, sometimes an enlargement on their course—the seminal vesicles—and a minute external pore situated on the side of the tail.
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. The segmentation is complete; one side of the hollow blastosphere invaginates and forms a gastrula. The blastopore closes, a new mouth and a new anus subsequently arising. The archenteron gives off two lateral pounchs and thus becomes trilobed. The middle lobe forms the alimentary canal; it closes behind and opens to the exterior anteriorly and so makes the mouth. The two lateral lobes contain the coelom; each separates off in front a segment which forms the head and presumably then divides again to form anteriorly the trunk, and posteriorly the tail regions. An interesting feature of the development of Chaetognaths is that, as in some insects, the cells destined to form the reproductive organs are differentiated at a very early period, being apparent even in the gastrula stage.
The great bulk of the group is pelagic, as the transparent nature of all their tissues indicates. They move by flexing their bodies. Spadella cephaloptera is, however, littoral and oviposits on seaweed, and the “Valdivia” brought home a deep-sea species.
The three genera are differentiated as follows:—
Sagitta M. Slabber, with two pairs of lateral fins. This genus was named as long ago as 1775.
Krohnia P. Langerhans, with one lateral fin on each side, extending on to the tail.
Spadella P. Langerhans, with a pair of lateral fins on the tail and a thickened ectodermic ridge running back on each side from the head to the anterior end of the fin.
The group is an isolated one and should probably be regarded as a separate phylum. It has certain histological resemblances with the Nematoda and certain primitive Annelids, but little stress must be laid on these. The most that can be said is that the Chaetognaths begin life with three segments, a feature they share with such widely-differing groups as the Brachiopoda, the Echinoderma and the Enteropneusta, and probably Vertebrata generally.
See O. Hertwig, Die Chaetognathen, eine Monographie (Jena, 1880); B.J. Grassi, Chetognathi: Flora u. Fauna d. Golfes von Neapel (1883); S. Strodtman, Arch. Naturg. lviii., 1892; N.M. Stevens, Zool. Jahrb. Anat. xviii., 1903, and xxi., 1905.