M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 337 



see, in all, distinct indications of the same fundamental plan, which 

 is probably best exemplified in the chain Salpa runcinata-fusiformis. 

 The variation affects the number, position, size, shape and histological 

 character of the eyes ; and so universal is the variation that the eyes of 

 no two species are alike. Salpa runcinata-fusiformis and Salpa Af ricana- 

 maxima most closely resemble each other, but even between these species 

 there are differences in the shape of the larger eye, the size of the optic 

 chamber, and the shape of the intermediate and pigment cells. 



This so great and so prevalent variation in the eyes of the several 

 species, which yet conform more or less closely to the same fundamental 

 type, offers an especially favorable opportunity for studying the relation- 

 ship between species. If the variation had been so great that the con- 

 formity to the type were lost in the adult, a careful embryological study 

 of the eye would be necessary to throw much light upon the relationship ; 

 but, since in the adult forms we have present both the variation and the 

 fundamental conformity, we can safely deduce certain evidence as to 

 relationship from a comparative anatomical study. This, in connection 

 with a careful study of the development of the most highly specialized 

 eye (that of Cyclosalpa pinnata), gives us good data from which to gain 

 evidence as to the phylogeny of the different species of salpidae. The 

 history of a single important organ which we have in this way obtained 

 can be reasonably taken as a source of definite and important evidence 

 as to phylogeny; though, of course, it is only evidence and cannot be 

 taken as decisive proof. A similar study of all the organs would be 

 necessary to fully establish the phylogenetic relations between the species. 



In salpa we are especially fortunate, since in the solitary form 

 we have a conservative member, while in the chain form we have a 

 more modified member of the species. It is as if an animal were 

 placed in such peculiar conditions that one portion of its body should 

 retain primitive characters while another portion of its body should 

 undergo the most decided changes. In salpa the divergent member 

 (chain individual) is so removed from the more conservative member 

 (the solitary salpa) that its modifications have unusually little effect upon 

 many features of this conservative member. The modifications of the 

 chain form do not, however, have any less bearing upon phylogenetic 

 questions than if they also affected the solitary form of the species. 

 Variations in the chain form and variations in the solitary form all 

 come from the same ultimate source, the germ plasm produced by the 

 solitary salpa. 



