20 



THE AMERICAN NATURALIST [Vol. XLVII 



provided with this structure, while in eight it is entirely 

 missing. The species comprising each group are listed 

 below : 



Species with Collarette Species without Collarette 



S. pulchra 

 S. sib ogee 



Those species having the collarette may be further 

 separated into diverse groups by means of the following 

 main characteristics: (1) Those in which the body is 

 transparent as contrasted with those in which it is 

 opaque, (2) those in which the collarette extends to the 

 ventral ganglion as contrasted with those in which it never 

 extends more than half way to the ganglion, (3) those 

 in which the anterior fin extends to the ventral gang- 

 lion as contrasted to those in which it does not, (4) those 

 having more than 50 per cent, of the posterior fin in 

 front of the tail-septum as contrasted with those having 

 more than 50 per cent, of the fin behind the tail-septum, 

 and (5) those in which the anterior fin is shorter than 

 the posterior fin as contrasted with those in which the pos- 

 terior fin is the shorter. Let it not be thought that these 

 are the only characteristics used to differentiate the 

 various species of Sagitta provided with the collarette. 

 Far from it! Many others of great specific importance 

 are made use of, but, if classified according to those just 

 enumerated, the most closely related 1 'couplets" remain 

 inseparable while those species not so closely related are 

 readily separated from each other. This may be graph- 

 ically represented by arranging these five pairs of con- 

 trasted characteristics into a series of 4 1 rows" and 



S. planktonis 

 S.hispida (robusta Doncaster) 

 S. tenuis 



S. bipunctata 

 S. decipiens 

 S. neglecta 

 S. regularis 

 S. ferox 



S. enflata 

 S. hexaptera 

 S. lyra 

 S. gazellce- 

 S. serratodentata 

 S. bedoti 



S. elegans 

 S. macrocephala 



