44 University of California Puhlicafions in Zoology. [Vol.8 



there is no constriction at the tail-septnm. Whether or not the 

 two species are in reality distinct I am not sure, but until I am 

 enabled to study S. decipicns with care I can not feel positive 

 concerning their relationship. 



Sagitta planktonis Steinhaus 



PI. 1, fig. 6; pi. 3, fig. 21; pi. 4, fig. 33; pi. 5, fig. 42; pi. 7. fig. 4(3; 

 pi. 8, fig. 48. 



Sagitta planktonis Steinhaus (1896), p. 39; Eitter-Zabony (1909c), 



p. 790. 

 Sagitta zetesios Fowler (1905), p. 67, (1906), p. 22. 



General Appearance 



On a white background S. pla)ikfonis appears very opaque. 

 The body proper assumes a light cream color, while the seizing 

 jaws and tail present a light brown tint. The ovaries when im- 

 mature are so slender and the muscles so thick that it is impos- 

 sible to see the ovaries with the naked eye. Even with a micro- 

 scope they are usually difficult to discern. On a black background 

 (pi. 1, fig. 6) the animal appears nearly white, the body proper 

 assuming a very pale bluish tint, while the head and ventral 

 ganglion appear somewhat whiter. The tail fin and both pairs of 

 lateral fins are readily seen with the naked eye, appearing as 

 transparent areas in decided contrast to the general opacity of 

 the body. 



Characters 



Body stout and rigid, retaining its form almost perfectly. Neck evi- 

 dent. Body widest behind center, at a i^oint from 50 to 63 per cent of 

 its length, gradually tapering toward head and tail. Lateral fields small. 

 Muscles strong and thick. No constriction at tail-septum. Collarette 

 present (pi. 7, fig. 46; pi. 8, fig. 48), extending posteriorly to anterior 

 fins. 



Anterior fin longer and narrower than posterior fin. It usually extends 

 to ventral ganglion, but rarely falls short by not more than 0.1 per cent 

 of the total length of the animal. Widest point of fin about 80 per cent 

 of its length from anterior end. Triangular in form, with the acute apex 

 continuous with the collarette. 



Posterior fin never extends to seminal vesicles. From 55 to 72 per 

 cent of fin in front of tail-septum. Point of greatest width in front of 

 tail-septum. Triangular in form, the obtuse angle being on the line of 

 greatest width. Interval from anterior to posterior fin varies from 7 to 

 12 per cent. 



Vestibular ridge (pi. 3, fig. 21) well develoi^ed, the papillae being 

 small and very irregular in form and position. They are not present 



