CHAEIOGNATHA COLLECTED BY STEAMER ALBATROSS. 



245 



SAGITTA HEXAPTERA d'Orbigny. 



Sagitta hexaptera d'Orbigny (1843), p. 140. — Fowler (1906), p. 11. — Ritter- 

 Zahony (1911), p. 7.— Michael (1911), p. 30. 



This species is the second niost abundant and frequent obtained 

 during the Philippine expedition, being represented by approximately 

 700 specimens. Most are large though immature, the largest speci- 

 men measuring 45 mm. in length. Curiously, both anterior and 

 posterior teeth in many specimens are entirely missing. My first 

 impression was that although not seen they must be present, but 

 careful dissection of several heads has made it certain that the teeth 

 are actually missing. This was not noticed in San Diego specimens, 

 two being the smallest number of either anterior or posterior teeth 

 recorded in my (1911) report. Again, Fowler (1906, p. 13) lists the 

 number of teeth in 42 specimens, one being the smallest number of 

 either anterior or posterior teeth recorded. Ritter-Zahony (1908, 

 p. 10), however, while recording three as the smallest number of 

 anterior teeth, gives four instances in which the posterior teeth in 

 individuals 33, 34, 36, and 38 mm. in length were missing. In 

 attempting to account for the peculiar variability in number of teeth, 

 Fowler (1906, p. 14) explains their absence in the Philippine speci- 

 mens. He says: 



I believe the explanation to lie mainly (perhaps not entirely) in the length and 

 slenderness of the teeth; many of them are probably torn out by the roots; certainly 

 many are broken off short, for their bases may be seen still in place. As an additional 

 weakness, the posterior teeth in older specimens often appear not to be attached to the 

 bony bar with which they are united in other species, but to lie at some distance from 

 it in a superficial plate of chitinous material. 



Table 3. — Measurements of Sagitta hexaptera. 1 



1 All measurements made in per cent of total length of animal. 



2 Per cent of posterior fin in front of tail-septum. 



3 The + signifies extension beyond anterior end of ganglion; the — signifies distance from posterior end 

 <of ganglion. 



