_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. [Illustration: FIG. 105.—_Spadella draco._ Dorsal view. × 12. (From Hertwig.) _a_, Cephalic ganglion; _b_, commissure between the cephalic ganglion and the ventral; _c_, eye; _d_, olfactory organ; _e_, alimentary canal; _f_, ovary; _g_, oviduct (the line goes a little beyond the duct); _h_, testis; _j_, vesicula seminalis.] _S. cephaloptera_ Busch is the smallest species of Chaetognatha, attaining at most a length of .5 cm. The body is not so transparent as in other species, and is of a yellowish colour. It has been found from the Orkney Islands to the Mediterranean. Strodtmann is of the opinion that the three species _S. mariana_ Lewes, _S. batziana_ Giard, and _S. gallica_ Pagenstecher differ from the above-named only in size, or that their description is too indefinite to permit of accurate {193}characterisation. He recognises three other distinct species: _S. pontica_ Uljanin, from the Black Sea; _S. marioni_ Gourret, from the Gulf of Lyons; and _S. draco_ Krohn, Mediterranean and Madeiran, and from the Canaries. Much confusion has been introduced into the classification of the Chaetognatha by Grassi,[230] who calls some—but not all—of what other writers term _Sagitta_, _Spadella_, and _vice versâ_. The following table was compiled by Strodtmann,[231] but I have incorporated in it two species recently described from Amboyna by Béraneck,[232] and called by him _Sagitta bedoti_ and _Spadella vougai_ respectively:— CHAETOGNATHA I. Two pairs of lateral fins; two rows of spines on the head; slender forms. (i.) Number of spines in posterior row greater than in anterior. _a._ Border of hooks smooth, their point not curved. α. No interval between the two fins on each side. 3.5 cm. long; 4-7 anterior spines, 8-11 posterior spines; olfactory organ lying entirely on the trunk. The anterior nerves of the ventral ganglion lie close to one another as far as the head.—SAGITTA LYRA. β. A distinct interval between the two fins on each side. _aa._ Adult animals large; hooks 6-7; anterior spines 3-4; posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral areas relatively larger.—SAGITTA HEXAPTERA. _bb._ Greatest length 1-2 cm.