26 University of California PuhJicafions in Zoology. [Vol. 8 



PART I. SYSTEMATIC 



KEY TO THE GENERA OF CHAETOGNATHA 



1. Two pairs of lateral fins. Two pairs of rows of teeth. Only 



slight epidermal thickening on body Sagitta. 



2. One pair of lateral fins, partly on body and tail. One pair of 



rows of teeth. No epidermal thickening behind head 



Eukrohnia. 



3. One pair of lateral fins, entirely upon tail-segment. Two pairs 



of rows of teeth. Prominent thickening of epidermis, ex- 

 tending from behind head to tail Spadella. 



KEY TO THE SPECIES OF SAGITTA 

 1. Species without collarette 2 



1. Species with collarette 9 



2. Shaft of seizing jaw serrated Sagitta serratodentata. 



2. Shaft of seizing jaw not serrated 3 



3. At least 50 per cent of posterior fin in front of tail-septum 4 



3. Less than 50 per cent of posterior fin in front of tail-septum 



Sagitta bedoti. 



4. Tail 28 to 40 per cent of total length Sagitta macrocephala. 



4. Tail 10 to 25 per cent of total length 5 



5. Vestibular ridge composed entirely of papillae 6 



5. Vestibular ridge provided with usual skeletal parts 8 



6. Tail 10 to 14 per cent of total length, mature ovary short, not 



over 7 per cent of total length Sagitta gazelle. 



6. Tail at least 15 per cent of total length, mature ovary long, 



over 20 per cent of total length 7 



7. Anterior fin shorter than posterior fin, with large interval be- 



tween them Sagitta hexaptera. 



7. Anterior fin longer than posterior fin; in the mature sjjecimens 



the fins are confluent Sagitta lyra. 



8. Body transparent. Top of shaft and base of point in seizing 



jaws converge toward edge of jaw Sagitta enflata. 



8. Bod}' opaque. Top of shaft and base of point parallel 



Sagitta elegans. 



9. At least 50 per cent of })osterior fin in front of tail-septum 10 



9. Less than 50 per cent of posterior fin in front of tail-septum 14 



10. Collarette extending to ventral ganglion Sagitta planktonis. 



10. Collarette never extending more than half way to ganglion 11 



11. Anterior fin extending to ventral ganglion 12 



11. Anterior fin never extends to ventral ganglion 13 



