No. 553] DISTRIBUTION OF THE CH2ET0GNATHA 19 



tween 32° 20' and 33° 30' N., and between the coast and 

 119° W. From this small area 68,962 specimens com- 

 prising ten species of Ohaetognatha have been collected, 

 and, as all depths between the surface and 350 fathoms 

 have been examined with horizontal closing nets, the 

 depth from which each specimen was obtained is known 

 with the nearest approach to certainty permitted by any 

 known method of collecting. As a critical analysis of 

 this data has been published elsewhere [see Michael 

 ('11)] reference must be made to that paper for the 

 methods, problems and details involved in determining 

 the vertical distribution of each species, so that, in the 

 following pages, only the fruits of that research bear- 

 ing directly upon the present subject-matter will be dis- 

 cussed. It will be shown (1) that of the most closely 

 related "couplets" of species only one has been taken 

 from the San Diego region, (2) that, of those species oc- 

 curring in this region, each has its own definite and 

 specific manner of vertical distribution, (3) that the most 

 diverse species (morphologically) have the most coinci- 

 dent vertical distribution, and (4) that, while several 

 species have sometimes been taken in the same haul, 

 rarely more than one was represented by sexually ma- 

 ture individuals. 



Relationships between the Species of Ch.etognatha 

 Adopting Eitter-Zahony's ('llfr) careful revision of 

 the Chastognatha as our starting point, the group be- 

 comes separable into six genera, Sagitta, Pterosagitta, 

 Spadella, Eukrohnia. Hcterokrohnia and Krohnitta. 

 Sagitta is represented by eighteen valid and four rather 

 doubtful species, Pterosagitta is represented by one, 

 Spadella by one, Eukrohnia by two, Heterokrohnia by 

 one and Krohnitta by two. 



Now the eighteen valid species of Sagitta fall into two 

 sharply contrasted groups by virtue of the presence or 

 absence of a collarette which is a conspicuous thickening 

 of the epidermis posterior to the head. Ten species are 



