40 



THE AMERICAN NATURALIST [Vol. XLVII 



This table shows similar and just as fundamental 

 morphological distinctions as those found between 8. 

 enflata and 8. neglect a. 



Group 3 of similar distributional species is composed 

 of 8. lyra and S. planktonis, and on referring to the key 

 (p. 36) we see that the two species are distinguishable 

 only by the fact that 8. lyra is of relatively common 

 occurrence above 150 fathoms. However, we find that 

 S. lyra is placed in Group 1 of similar morphological 

 species, while S. planktonis is placed in Group 2. The 

 following table reveals the main morphological differ- 

 ences between the two species. 



TABLE X 



Structural Differences between Sagitta lyra and Sagitta planktonis 



Collarette. Entirely wanting. Massive, extending to 



ventral ganglion and 

 anterior fin. 



Translucent, nearly trans- Exceptionally opaque, 

 parent. Firm and rigid, retain- 



0dy - Tumid, but not retaining ing its form almost 



its form well. perfectly. 

 Muscles. Weak and thin. Strong and thick. 



Length of anterior fin. 31.4-44.5 per cent, of 18.8-27 per cent, of total 

 total length of animal. length of animal. 



Extent of posteri 



15.6-24.8 J 

 tal lengt 



Here again we find that there is no question concerning 

 the great morphological difference between these two 

 similar distributional species. 



