NOTES ON THE IDENTIFICATION OF CH/ETOGNATHA. 'J'J 



allow sufficient variation to feel quite safe, except in the case of 

 Sagitta bipunctata (7.9 per cent.) and possibly 5. serratodejitata 

 and ^. niacroccpJiala (7.5 per cent.). 



The limits of the anterior fin as tested by the ventral gang- 

 lion, and the position of greatest width are of some value to the 

 systematist, in individuals well preserved. In Sagitta ferox, S. 

 jieglecta, S. piilcJwa, S. zuhartotii, S. serratodentata, S. siboga, S. 

 zetesios, and ^. plaiictonis the fin extends to the ganglion ; in 

 Sagitta oifiata, S. hexaptcra, and S. bipunctata the fin is remote 

 from the ganglion. The remaining species occupy various points 

 between these two extremes. Until the extent of this variation 

 is better known, for each species, separation cannot be rendered 

 any more definite, by this criterion. 



The posterior fin is more instructive. The position of greatest 

 width, as tested by the tail septum ; the proportion of posterior 

 fin in front of the septum ; the proximity of the fin to the vesic- 

 ulse seminales, are all serviceable characters. The table shows 

 the grouping of the species in this matter. 



The corona ciliata has been considerably used in the past. 

 Length and width in proportion to the total length, shape and 

 location, how much on head, how much on body, — these form 

 the important features. The great difficulty in using this char- 

 acter lies in the fact that it is very rarely present in preserved 

 material. With living material the corona might have consider- 

 able significance, but with formalin material nearly all specimens 

 have apparently lost the structure. 



The collarette and lateral fields are of some utility. The col- 

 larette or neck fin is an expansion of the ectoderm in the re- 

 gion of the neck and appears as a constant specific character. 

 Some species are always provided with it as Sagitta siboga, S. 

 ferox, S. ncglecta, S. regidaris, S. pjilclwa, S. robiista, S. zetesios, 

 and 6". decipiens. It is absent in the remaining species. The 

 lateral fields are those areas between the muscles so that, in gen- 

 eral, the presence of large lateral fields is co-existent with weak 

 muscles. Species in this category are very often flabby and 

 transparent in formalin. Formalin acts upon the muscles caus- 

 ing opacity and firmness so that the species with strong longi- 

 tudinal muscles are readily separated from those with weak mus- 



