THE SALPIDAE: A TAXONOMIC STUDY. 



By Maynard M. Metcalf, 



Of the Orchard Laboratory, Oberlin, Ohio. 



With the Assistance of Mary M. Bell. 1 



In studying the interrelationships of the Salpidae, one finds the 

 greatest significance attaching to three sets of organs — the gut, the 

 nervous system, and the muscles, the symmetry or asymmetry of the 

 body and its muscles being a somewhat distinct point worthy of con- 

 siderable emphasis. Other features will occasionally be mentioned, 

 but are of minor importance. In all of these particulars the solitary 

 form is readily seen to be the more conservative, while the aggregated 

 individuals diverge far more. This more plastic character of the 

 aggregated form causes it to present a larger body of phenomena 

 in which evidence of relationship may be seen. In studying the 

 Salpidae we are fortunate in having in the life cycle of each species 

 a conservative member, the solitary form, showing in larger measure 

 ancestral traits, and a more divergent member, more sensitive to 

 modern trends in the evolution of the family. 



In most groups of animals the nervous system is found to be a con- 

 servative organ system, not liable to much modification from super- 

 ficial influences. It is therefore, in general, one of the best sources of 

 taxonomic evidence. In the Salpidae, on the other hand, some fea- 

 tures of the nervous system are found to be quite inconstant. For 

 instance, the number of the nerves radiating from the ganglion is 

 found to be variable, the grouping of the fibers into nerves being dif- 

 ferent in different individuals, or even on the two sides, right and left, 

 of the ganglion in the same individual. This inconstancy is probably 

 connected with the fact that the nervous system of the Salpidae is 

 secondarily simplified ("degenerate"). There are outgrowths from 

 the ganglia of certain species, especially in the aggregated forms, and 

 the character of these is constant within the species and of taxonomic 

 significance. 



1 This paper is written by Maynard M. Metcalf. He has had the assistance of Miss Bell in studying 

 the anatomy of the Cyclosalpae, and Traustcdtia, and all of the original drawings of these forms were made 

 by her. Numerous other drawings, as noted in each instance, have been made by Mr. Hoyt S. Hopkins 

 usually after careful independent study, and I wish to acknowledge my indebtedness to him not only for 

 the drawings, but also for his observations, which have checked up and have supplemented my own. 



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