CLASSIFICATION 



191 



through a very short and rapid development, 

 known to be phosphorescent. 



Classification. The features of the Chae- 

 tognatha which have most systematic value 

 are the size of the adult, the relations of the 

 length to the breadth, and of the three divi- 

 sions to one another ; the size, number, and 

 position of the lateral fins,, and of the hooks 

 and spines on the head ; the thickness of the 

 epidermis, -and the structure of the olfactory 

 organ ; and, finally, the form of the reproduc- 

 tive organs. 



Strodtmann, 1 who gives the latest and most 

 complete account of the species of Chaeto- 

 gnatha, arranges them under three genera, 

 which he characterises as follows : 



(i.) Sagitta Slabber. Two pairs of lateral 

 fins, two rows of spines on the head. The 

 lateral thickening of the epidermis absent or 

 insignificant. 



Under this genus are included nine definite 

 species and five others S. gracilis Yerrill, 

 S. elegans Yerrill, S. darwini Grassi, S. dip- 

 tera d'Orbigny, and S. triptera d'Orbigny 

 whose position, owing to the inadequacy of 

 their description, is of doubtful validity. 



The distribution of the other species may 

 be mentioned. S. hexaptera is the largest 

 Cliaetognath known, and reaches in the adult 

 stage a length of 7 cm. It is very widely 

 distributed, being found in practically all the 

 temperate and warm seas, usually at the sur- 

 face of the water, though at times it is found 

 at a depth of one metre, or even deeper. " S. 

 lyra, Mediterranean, very rare. S. tricuspi- 

 data, widely distributed. 8. magna, Mediter- 

 ranean and Madeiran, living at the surface. 

 & hipunctata, the most frequently described 

 form, smaller than the preceding species, 1-2 



1 Archiv Naturg. 58 Jahrg. Bd. i. 1892, p. 333 



are 



\ 



Fig. 104. Sagitta hax- 

 aptera. Ventral view, 

 x 4. ( From Hert- 

 wig.) a, Mouth ; 

 b, hooks ; c, anterior 

 septum ; d, aliment- 

 ary canal ; e, com- 

 missure from the 

 brain to the ventral 

 ganglion ; /, ventral 

 ganglion ; g, ovary ; 

 h, oviduct ; i, pos- 

 terior septum ; j, 

 testis ; k, vesicula 

 seminalis. 



