bb. Greatest length 1-2 cm.
αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.
ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.
γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.
δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.
εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.
b. Edge of hooks toothed and their point bent round; hooks 6-8; anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender; conspicuously projecting vesiculae seminales.—Sagitta serratodentata.
(ii.) Number of the spines in posterior row smaller than in anterior.
a. Anterior spines 3; posterior spine 1; hooks 8; length 3.5 cm.—Sagitta tricuspidata.
b. Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1 cm.; tail ⅕ of the total length.—Sagitta magna.