FRESHWATER INVERTEBRATE DISTRIBUTION 229 



even though definitive data are lacking for the Oligochaeta, Gas- 

 trotricha, and Nematoda, and their inclusion here is based on 

 information gathered primarily from European sources. The next 

 five taxa of crustaceans form a natural group featured by a high 

 percentage of endemic species in the West (with the possible ex- 

 ceptions of the Copepoda and Cladocera). The following six taxa 

 are a heterogeneous group which, with the exception of the Gastro- 

 poda, are poorly known in the West, chiefly because of the lack of 

 intensive and systematic collecting. The last four taxa listed in 

 the first column are all relatively poor in species in the West, but 

 nevertheless most of the species found there are endemic. 



6. The last point I should like to make is concerned with the 

 increasingly important role being played by man in the distribution 

 of freshwater invertebrates in the West. Residents of California, 

 for example, are well aware of the nuisance and economic importance 

 of cambarine crayfishes that were intentionally introduced here 

 many years ago. In Colorado we are seeing the same process, 

 although the end results will probably not be so serious; cambarine 

 crayfishes have been introduced into certain Colorado mountain 

 lakes, and in some such habitats they have become abundant and a 

 welcome addition to the trout diet. In addition, however, they have 

 been introduced into the Western Slope drainages, and in a few 

 irrigation ditches they are occasionally a nuisance because of their 

 burrowing habits. 



Aquarium enthusiasts are often responsible for setting up new 

 centers of distribution for both bivalve and univalve mollusks. 

 Many species native to the eastern states are well adapted to home 

 aquarium use, and, of course, these are commonly shipped west as a 

 part of the thriving aquarium business. Frequently, however, 

 when the hobbyist is tired of his aquarium, he empties it into the 

 nearest pond or lake, and we may thus have a new mollusk popul- 

 ation established under natural conditions. 



Fishermen are consciously accomplishing the same ends. We 

 have seen fishing parties from Nebraska, for example, bringing 

 milk cans full of vegetation and the associated invertebrates to 

 Colorado. These they commonly empty into mountain lakes on 

 both sides of the Continental Divide. Furthermore, state fish and 

 game departmants frequently do exactly the same thing when 

 they transport large quantities of invertebrates and vegetation from 



