160 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



portion of the large eye is missing, while in the aggregated Pegea and 

 Traustedtia no inversion of the eye occurs. Brooksia, in its affinities, 

 leans perhaps toward the Sphaerodaea, the aggregated zooid being 

 very asymmetrical, more so than in Ritteria, though its gut is de- 

 scribed as less compact than is the gut in Apsteinia or Salpa. Its 

 aggregated zooids show marked resemblance to those of the most 

 highly modified of the Cyclosalpae, C. virgula. The remarkable 

 anterior protuberance in the solitary Brooksia seems to resemble in 

 a general way the anterior protuberances seen in the aggregated 

 zooids of all the subgenus Salpa. 



The most primitive of the Salpidae — the Cyclosalpae symmetri- 

 cales — I suggest, may owe the absence of symmetry in their aggre- 

 gated zooids to their arrangement in the form of a wheel. The 

 presence in the eye of Cyclosalpa pinnata of a slight though constant 

 asymmetry (see p. 10) suggests that even the symmetrical Cyclosalpas 

 may have been at one time more asymmetrical. The wheel-like 

 grouping of their aggregated zooids is probably a secondary character. 

 The higher Cyclosalpas have very asymmetrical aggregated zooids, 

 so also do the more primitive members of the two major divisions 

 derived from the Cyclosalpas, but the most aberrant members of 

 the family Salpidae show decreasing asymmetry in the aggregated 

 forms. All, however, show some asymmetry, however disguised. 

 The position of the large eye in the aggregated zooids is asymmetrical 

 in all of these aberrant species. 



In the group Salpidae, hybridization is not known. Each species 

 is clearly demarcated. The two subspecies, Cyclosalpa pinnata polae, 

 and Pegea confederate bicaudata, are definite. Salpa fusiformis, form 

 aspera, seems hardly to be worthy of rank as a subspecies, for it com- 

 pletely intergrades with the species proper. On the other hand the 

 tuberculate variety of Salpa maxima is not known to intergrade with 

 the species itself. 



The question of treatment of genera and subgenera, species, and 

 subspecies is one a little troublesome to decide. If we are to define 

 and use the term species with strict scientific accuracy I suppose 

 every persistent mutant, however slightly it diverges, must be 

 classed as a distinct species. Any other classification would be 

 purely subjective, dependent upon the judgment of the student as 

 to the importance or nonimportance of the diversities observed. 

 Species and genera, as they are employed in classification, are largely 

 conventional. They must be so, for the recognition in our taxonomic 

 systems of every distinct mutant would be utterly impossible, and 

 there is no criterion for genera which removes the personal equation 

 of the observer. The question then, in its practical aspects, is one 

 of convenience and of the best expression of probable relationship. 

 I have chosen to treat as subgenera rather than genera the groups 



