Fig. 102.—Head of Sagitta bipunctata. A, Dorsal view; B, ventral view. × about 33. (From Hertwig.) A, a, spines; b, nerves to lateral cephalic ganglia; c, hooks; d, cephalic ganglion; e, olfactory nerve; f, optic nerve; g, hood; h, commissure to ventral ganglion; j, olfactory organ: B, a, c, and g as in A; k, mouth.
The Chaetognatha are hermaphrodite, and carry the female organs in the trunk, the male in the tail. In a mature specimen the two ovaries occupy almost all the space in the trunk between the alimentary canal and the skin, and each is supported by a narrow lateral mesentery. The ovary is traversed by a oviduct which often contains spermatozoa; it is not clear how the eggs make their way into the oviduct, which seems to have no internal opening and to act largely as a receptaculum seminis. The oviducts open externally on the upper side at the base of the lateral fin, close to the junction of the tail and the trunk.
The cavity of the tail is divided into two lateral chambers by the extension backward of the median vertical mesentery. In each of these a testis and a vas deferens are found. The testes are solid ridges formed by the growth of the lining cells of this part of the body-cavity; the cells mature into spermatozoa, which break off and float freely in the coelomic fluid. At the breeding season the whole tail may be crowded with masses of spermatozoa, which are kept in a more or less regular circulation by the ciliated cells lining the body-wall. The vas deferens opens internally into the space where the spermatozoa lie, and at the other end into a vesicula seminis, which opens to the exterior. The position of the latter structure varies, and is of some systematic value.
The eggs are laid in the water and as a rule float at the surface of the sea. Spadella cephaloptera is, however, an exception to this rule, as it attaches its eggs by means of a gelatinous stalk to sea-weeds. The segmentation of the ovum is regular, and gives rise to a two-layered stage or gastrula, which opens by a pore, the blastopore. This does not, however, become the mouth, but closes up and the mouth arises at the opposite pole. Perhaps the most interesting feature of the development of Sagitta is that the cells destined to form the reproductive organs separate from the other cells of the embryo at a very early date, whilst it is still in the gastrula stage. There is no larval form, but the young hatch out from the egg in a state resembling the adult in all respects but that of size.
Fig. 103.—Spadella cephaloptera. Dorsal view. x 30. (From Hertwig.) a, Cephalic ganglion; b, commissure to ventral ganglion; c, olfactory organ; d, alimentary canal; e, ovary; f, oviduct; g, testis; h, vesicula seminalis.
Habits.—The Chaetognatha are essentially pelagic, and resemble many other creatures that dwell at the surface of the ocean in being almost completely transparent. Most species have been taken far out at sea, but some are perhaps rather more numerous near the coast, and one species, Spadella cephaloptera, is littoral. They swim by means of muscular movements of the whole body; the fins have no movement of their own, and seem to serve as balancers, and not as locomotory organs. Although usually found at the surface of the water, many species have been taken at considerable depths. Chun[[225]] states that they are found in countless numbers at depths of from 100 metres to 1300 metres. The commonest species at these depths are Sagitta hexaptera and Sagitta serratodentata. Sagitta bipunctata, according to the same authority, confines itself to the surface. Whether the change of depth is diurnal, or whether it has any relation to sexual maturity, or to any other cause, has not been satisfactorily determined.
The food of the Chaetognatha consists of floating diatoms, Infusoria, small larvae, and such Copepods as Calanus finmarchicus, and small Amphipods as Phoxus plumosus.[[226]] At times they also devour small larval or post-larval fishes, and owing to their incredible numbers, they doubtless do considerable damage to sea fisheries. It is also recorded that they eat one another, and specimens have been taken which have ingested the whole body of another Sagitta except the head, which hangs out of the mouth of the eater, and gives it the appearance of a double-headed monster.[[227]] It has been said that they attack hydroid polypes, but here at any rate they do not have it all their own way. Masterman[[228]] has figured the apical group of five polypes of Obelia, three of which are engaged in ingesting as many young Sagitta.
They exist in incredible numbers; Grassi describes the surface of the sea at Messina on certain days as being literally covered with them, and they must form the food supply of numerous animals which prey upon the pelagic fauna. The immense number of individuals is probably accounted for to some extent by the fact that they lay eggs all the year round, and pass through a very short and rapid development. They are not known to be phosphorescent.
Classification.—The features of the Chaetognatha which have most systematic value are the size of the adult, the relations of the length to the breadth, and of the three divisions to one another; the size, number, and position of the lateral fins, and of the hooks and spines on the head; the thickness of the epidermis, and the structure of the olfactory organ; and, finally, the form of the reproductive organs.