194 CHAETOGNATHA chap, vii 



ee. 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 l - 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 \ 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. 



(ii.) Lateral extension of the epidermis not so conspicuous, and the cells 

 composing it smaller. Tactile organs in little depressions. Trans- 

 verse as well as longitudinal muscles in the trunk. Adhesive cells 

 on the ventral surface of the body. No interval between the lateral 

 fins and the tail fin. Two papillae on the head-hood elongated 

 into club-shaped tentacles. Hooks 8-9, slightly serrated ; anterior 

 spines 3-4 ; posterior spines 3-4. Spadella cephaloptera. 



(iii.) Similar to the last-mentioned species, but the tail segment is larger 

 than the trunk ; in the above it is of the same size. No adhesive 

 cells. The fins are covered with papillae, and with a number of 

 serrated spines pointed at both ends. Spadella pontica. 



(iv.) Tactile organs and adhesive cells are unmodified epidermal cells. 

 Anus dorsal. Orifice of oviducts ventral. No olfactory organ. 

 Epidermis colourless. Lateral fins without rays. A pair of ganglia 

 at the postero-lateral angle of the brain. Spadella marioni. 



(v.) Tactile organs well developed on the head, trunk, and fins ; tail 

 segment a little shorter than the trunk. Body short, length 3-4 

 mm. Hooks 9 ; anterior spines 4-5, posterior spines 6-7. 

 Spadella vougai. 



