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. [Illustration: 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 {190}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_.