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. {194}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. II. One pair of lateral fins lying on the trunk and tail; one row of spines; body slender; epidermis not thickened. (i.) Hooks 8-9, bent like an elbow at the point, serrated in the young; 20-25 spines in a row; ovary reddish; length 3-4 cm.—KROHNIA HAMATA. (ii.) Hooks 8, broad at their base but very sharply pointed; spines in a curved row, about 18, with a constriction below like the neck of a bottle; body thin; length 1-1.5 cm.—KROHNIA SUBTILIS. III. One pair of lateral fins, these lie on the tail; body relatively very broad in consequence of the thickening of the epidermis lying behind the head; two rows of spines; greatest length 1 cm.; tail and trunk usually the same length. (i.) A great extension of the epidermis behind the head, consisting of very large cells; amongst these, at the level of the ventral ganglion, lies a bundle of stiff hairs; tactile organ on papillae; hooks 9-10; anterior spines 6-8; posterior spines 12-18.—SPADELLA DRACO.