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 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.