218 FRESH-WATER NEREIDS FROM THE PACIFIC COAST AND HAWAII. 



D. baicalensis are of the same species; but this is evidently not the case, as seen by 

 comparing the uncini, as they are figured by Leidy and by Nusbaum. 



All thoroughly established fresh-water forms must necessarily breed in their 

 new habitat. Leidy ('84) found this to be the case with Manayunkia, in which, 

 as usual with fresh-water invertebrates, the development is direct. The dis- 

 covery of the young of Nereis limnicola in Lake Merced in May, 1899, these young 

 having only 18 to 20 somites, but well beyond the metamorphosis if there was 

 one proves that this species breeds in fresh water. Unfortunately, younger 

 stages have not yet been obtained. That fresh-water breeding is not always pos- 

 sible for euryhaline polychsetes is evident from the observations of Mendthal 

 ('90) on Nereis diversicolor. This species occurs in the "Frisch Haff" near Konigs- 

 berg, Prussia. The Frisch Haff, although having intermittent connection with the 

 Baltic Sea, is almost perfectly fresh, and has a fresh-water fauna and flora. N. diver- 

 sicolor, perhaps the most euryhaline of all European nereids, is the only polychsete 

 found there, and according to Mendthal it does not breed in the Frisch Haff, where 

 only adults are found. N. diversicolor cannot therefore be regarded as fully estab- 

 lished, even as a brackish-water inhabitant, although as an adult it can apparently 

 live indefinitely in water of very weak salinity.* 



A significant fact in the physiology of certain thoroughly acclimated, and, so 

 far as known, exclusively fresh- water-dwelling Polychseta, is that they can easily 

 be made to live in ordinary sea- water. It is only necessary to make the transition 

 a gradual one. I have subjected Nereis limnicola to the experiment of passing 

 it through a series of aquaria of ever-increasing salinity until after a few days it was 

 living in undiluted sea-water without apparent discomfort. The same experiment 

 has been tried with equal or greater success on Manayunkia by Dr. J. Percy Moore, 

 who has kindly permitted me to make use of his unpublished results. Manayunkia 

 endured the change with ease and lived for four months in pure sea-water. It is 

 obvious that the complete success or partial failure of such experiments may be 

 dependent upon the food-supply. 



The reverse experiment of passing marine or brackish-water forms into fresh 

 water has often been tried with more or less success. Eisig ('87, p. 798), by making 

 the dilution very gradual, was able in the course of four months to bring Capitella 

 capitata into a medium consisting of four parts of sea-water and ten parts of fresh 

 water, the reduction in specific gravity being from 1.034 to 1.0088. The worms did 

 not long endure so great a reduction of the salinity, so this form cannot be reckoned 

 as among the strongly euryhaline species. Spio fuliginosus under the same condi- 



* The Frisch Haff is stated by Mendthal to contain only .0035 per cent of chlorine. 



