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 {191}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. Strodtmann,[229] who gives the latest and most complete account of the species of Chaetognatha, arranges them under three genera, which he characterises as follows:— (i.) _Sagitta_ Slabber.—Two pairs of lateral fins, two rows of spines on the head. The lateral thickening of the epidermis absent or insignificant. Under this genus are included nine definite species and five others—_S. gracilis_ Verrill, _S. elegans_ Verrill, _S. darwini_ Grassi, _S. diptera_ d'Orbigny, and _S. triptera_ d'Orbigny—whose position, owing to the inadequacy of their description, is of doubtful validity. [Illustration: FIG. 104.—_Sagitta hexaptera._ Ventral view. × 4. (From Hertwig.) _a_, Mouth; _b_, hooks; _c_, anterior septum; _d_, alimentary canal; _e_, commissure from the brain to the ventral ganglion; _f_, ventral ganglion; _g_, ovary; _h_, oviduct; _i_, posterior septum; _j_, testis; _k_, vesicula seminalis.] The distribution of the other species may be mentioned. _S. hexaptera_ is the largest Chaetognath known, and reaches in the adult stage a length of 7 cm. It is very widely distributed, being found in practically all the temperate and warm seas, usually at the surface of the water, though at times it is found at a depth of one metre, or even deeper. _S. lyra_, Mediterranean, very rare. _S. tricuspidata_, widely distributed. _S. magna_, Mediterranean and Madeiran, living at the surface. _S. bipunctata_, the most frequently described form, smaller than the preceding species, 1-2 {192}cm. in length, widely distributed, and as a rule living near the coast line. _S. serratodentata_, Mediterranean. _S. enflata_, on the surface of the sea, Mediterranean and Madeiran. _S. minima_, a very small species, 1 cm. in length, Mediterranean. _S. falcidens_, Atlantic, off the coast of New Jersey. (ii.) _Krohnia_ Langerhans.—A single lateral fin extending on to both trunk and tail segment, no lateral epidermal extensions behind the head, only one row of spines on the head. Trunk longer than the tail.