No. 553] DISTRIBUTION OF THE CHJETOGNA Til A 23 



Looking to the other genera we find Eukrohnia com- 

 posed of two species (E. hamata and E. foivleri), Kroh- 

 nitta of two (K. subtilis and K. pacifica), and Hetero- 

 krohnia, Pterosagitta and Spadella of one each. Now 

 E. hamata and E. fowled form an exceedingly closely 

 related "couplet," but only the former is known to occur 

 in the San Diego region. Again, K. subtilis and K. 

 pacifica are so nearly alike that it is very difficult to de- 

 scribe their differences although they are probably valid 

 species. Yet, only the first has been found in California 

 waters. Of the three remaining genera Heterokrohnia 

 and Spadella are not represented in our collections, and 

 Pterosagitta by only one individual of its single species 

 P. draco. 



In so far, therefore, as the relationships among the 

 Chsetognatha have been correctly interpreted, it is evi- 

 dent that, except for the occurrence of both 8. enfiata 

 and S. hexaptera, there is no instance of two of the most 

 closely related species having been taken from the San 

 Diego region. 



General Distribution of the "Couplets" 

 Having pointed out that only one of a "couplet" of 

 the most closely related species occurs in the San Diego 

 region, it will be interesting to ascertain to what extent 

 the same relation holds in other parts of the world. 

 Furthermore, wherever both members of a "couplet" 

 are recorded from the same vicinity it will be to the 

 point to determine, if possible, to what extent their dis- 

 tribution within the area is coincident or isolated. 



S. neglecta and S. regularis 

 The members of this "couplet" may be designated as 

 warm water, epiplanktonic species whose northern and 

 southern limits of distribution are 35° N. and 9° S. The 

 highest surface temperature recorded in connection with 

 their capture is 29° C. and the lowest 15°.5 C. They were 

 both originally described by Aida ('97) from Misaki 



