272 



CH^TOGNATHA 



Key to the American genera of Chcetognatha: 



o x Two pairs of fins besides the caudal fin 1. SAGITTA 



a 2 One pair of fins besides the caudal fin. 



&! Fins near the middle; body slender, with 1 row of teeth 2. EUKROHNIA 



& 2 Fins near the tail ; body broad, with 2 rows of teeth 3. PTEROSAGITTA 



1. SAGITTA Quoy and Gaimard. Slender worms with 2 

 pairs of lateral fins : about 25 species. 



S. elegans Verrill (Fig. 443). Length up to 3 mm.; 9 to 

 12 oral hooks; anterior teeth 4 to 8; posterior teeth 6 to 9: 

 North Atlantic. 



S. hexaptera D'Orbigny. Length 6 mm.; oral hooks 5 

 to 10; anterior teeth 3 to 4; posterior 

 teeth 3 to 5: Martha's Vineyard; cos- 

 mopolitan. 



2. EUKROHNIA Ritter-Zahony. A 

 single lateral fin on each side near mid- 

 dle of body; body slender: 3 species. 



E. hamata (Mobius). Length 4 

 mm.; oral hooks 8 

 to 10; 15 to 28 

 teeth in a single 

 row ; ovary reddish : 

 cosmopolitan; Mar- 

 tha's Vineyard. 



3. PTEROSAGITTA Costa. Body broad; a single 

 pair of lateral fins near the tail; an expansion of 

 the integument extending along the side of the body 

 in front of each lateral fin to the head : 1 species. 



P. draco (Krohn) (Fig. 444). Length 10 mm.; 

 oral hooks 4 to 10; anterior teeth 6 to 9; posterior 

 teeth 12 to 18; forward of the middle of the body 

 on each side is a bundle of long setae : cosmopolitan. 



Fig. 443 Sagitta elegans (Zahony). 

 A, entire animal ; B, head. 1, pre- 

 hensile hooks ; 2, teeth ; 3, mouth ; 

 4, ventral ganglion ; 5, female gen- 

 ital organs ; 6, anus ; 7, female 

 genital pore ; 8, male genital pore. 



Fig. 444 

 Pterosagitta draco 



(Cambridge 

 Natural History). 



SUBPHYLUM 8. SIPUNCULOIDEA. 



Marine worms which, together with the Echiurida, are often included 

 in a class of the Annelida called the Gephyrea. This is a name given 

 them by Quatrefages (1847) to signify the belief that they bridge the 

 gulf between the Annelida and the Echinodermata, the earlier zoologists 

 having grouped them with the holothurians. The total lack of segmen- 

 tation, however, at any period of the life of most of the Sipunculoidea, 

 and the absence of metameric organs, have made it necessary to remove 

 them from the Annelida, although the fact that they pass through the 



