6 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The form of the horseshoe-shaped eye is much the same in the soli- 

 tary individuals of all species, therefore little taxonomic data is 

 found here. In the solitary forms of several species there are 

 accessory eyes, or accessory portions of the horseshoe-shaped eye, 

 and the character of these should be considered. But it is in the 

 aggregated forms that the eyes furnish the chief taxonomic evidence. 

 This evidence will be emphasized in the present paper. I have 

 studied the eyes of the solitary and aggregated forms of all species 

 of Salpidae except Brooksia rostrata, Apsteinia magalhanica, Ritteria 

 retracta, R. picteti, R. amboinensis, Thalia longicauda, and Traus- 

 tedtia, in none of which have the eyes of the aggregated forms been 

 adequately studied for any profitable comparisons, except in the 

 case of Traustedtia multitentaculata, for which Traustedt (1893) and 

 Dobcr (1912) give meager data, which, however, will be seen to be 

 significant. Goppert (1892) has made very accurate studies of the 

 eyes of five species. Redikorzefl: (1905) carefully reviews the struc- 

 ture of the eyes of three species, and Dober (1912) makes casual men- 

 tion of the eyes of a number of species, figuring surface views with 

 little detail. Citations will be made from all these sources. 



Accessory, smaller eyes are present in the aggregated forms of a 

 number of species. The number, form, position and histological 

 condition of these accessory eyes, as well as the form, position and 

 histological condition of the chief eye, will furnish usable evidence. 



Divergent as are the eyes in the several subgenera and species of 

 Salpidae, their structure is constant within the species, and so fur- 

 nishes an abundance of taxonomic evidence. This evidence is of 

 especial value, because the changes that have occurred in the phylo- 

 geny of these eyes have been so largely degenerative rather than 

 adaptive. Adaptive divergence and convergence can be left out of 

 consideration in connection with these degenerating organs. Such 

 changes as have occurred in these eyes in their recent phylogeny 

 probably have been due to trends within each species and not to 

 adaption to environment. The natural affinities of the species are 

 therefore probably more clearly expressed than they could be were 

 the freedom to change the structure of the eyes limited by relations 

 of utility. 



Reference to the chart on page 158 will show the classification of 

 species which I have adopted, and it should be studied a few moments 

 at this point, if my nomenclature is to be understood in the further 

 reading of this paper. There seems little choice as to the purely 

 academic question whether the subdivisions of the Salpidae given in 

 this chart be taken as genera or subgenera. The argument from con- 

 venience would favor treating them as subgenera. Relationships 

 are indicated with equal clearness according to either usage. 



I have been fortunate in having for study, in addition to my own 

 considerable, collections, all the collections of Salpidae made by the 



