FRESH-WATER NEREIDS FROM THE PACIFIC COAST AND HAWAII. 217 



It is seen from the foregoing list that the euryhaline Polychseta are not numer- 

 ous, considering that many hundreds of species belong to this order. Furthermore, 

 we discover the interesting fact that notwithstanding the euryhaline forms have 

 been found in nearly all parts of the world and in both tropical and temperate lati- 

 tudes, they belong to only five out of the forty-odd families of the Polychaeta; and of 

 the five, three have each but a single species in fresh or brackish water. Two families, 

 the Nereidse and the Sabellidse, furnish 87.5 per cent of all known euryhaline poly- 

 chsetes; and the Nereidse alone afford 15 out of a total of 24, or 62.5 per cent of the 

 entire list! That this ratio is being more than maintained in these days of active 

 faunistic and experimental investigation is demonstrated by the fact that of the 14 

 euryhaline species added to the list during the past decade, there are 9 species of 

 Nereidse and 4 of Sabellidse, nearly 93 per cent of the entire number added. 



We may therefore safely regard these two families as having the strongest ten- 

 dency towards euryhalinism of all the Polychseta. Yet neither of them has been 

 held as in the remotest likelihood the progenitor of the Oligochseta or of any fresh- 

 water invertebrates. Whatever the duration of their residence in lakes and streams, 

 the fresh-water members of these widely different families have not so far diverged 

 from the parent stock as to create the slightest doubt regarding their affinities. With 

 the possible exception of Caobangia, none of them requires the establishment of any 

 group higher than the genus. According to our present knowledge, on the other 

 hand, there are almost as many fresh-water genera as there are thoroughly estab- 

 lished fresh-water species. In other words, to borrow a convenient term from 

 the ornithologists, fresh-water polychsetes are apt to belong to monotypic genera. 

 Thus we have Caobangia, Eisigella, Lycastoides, Manayunkia, and Dybowscella, 

 most of them represented by a single species and none of them with any known marine 

 representatives. So far as known, every species belonging to these genera is strictly 

 limited in its natural habitat to fresh water. It must be admitted, however, that our 

 present knowledge is very incomplete, and certain experimental data indicate that 

 we are likely to find these forms living somewhere in marine or brackish-water habi- 

 tats. 



A point of considerable interest is the close relationship between Manayunkia 

 and Dybowscella. There is here no need of separate genera a fact that Zykoff (:01) 

 has already pointed out. There are three closely allied species: one, Manayunkia 

 speciosa, long ago described by Leidy ('59, '84), Potts ('85), and Foulke ('85) as 

 occurring in perfectly fresh water in the Schuylkill River, and in a pond near Absecom, 

 New Jersey; and two, Dybowscella baicalensis and D. godlenskii, described by Nus- 

 baum (: 01), living in Lake Baikal. Zykoff is even of the opinion that M. speciosa and 



