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.
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 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.
Krohnia has but two species: K. hamata Möbius, with a length of 3-4 cm., found in the North Atlantic and at considerable depths, 200 to 300 fathoms; and K. subtilis Grassi, 1.5 cm. long, with an extraordinary slender body and a relatively large head, found at Messina, but very rare; as a rule only one specimen has been found at a time.
(iii.) Spadella Langerhans.—A single pair of lateral fins; these are situated on the tail segment. Behind the head a thickening of the epidermis extends down each side of the body to the fin, or even farther. Two rows of spines on the head. Small animals, not longer than 1 cm.
