4S0 MONTGOMERY. [Vol. XI. 



in very many localities, from which they have not yet been 

 reported. But, nevertheless, judging from the fact that these 

 forms are in our present state of knowledge far fewer numeri- 

 cally than the marine forms, and that additions to the latter 

 are constantly being made, it is correct to conclude that by far 

 the greater number of species are marine. Accordingly we 

 have to consider the seas as the original home of the nemer- 

 teans, and those forms occurring in freshwater or on land we 

 must regard as marine species which have adapted themselves 

 to new environments. 



Freshwater species may have been produced in two ways. 

 In the first place, at a locality where a river empties into the 

 sea, certain individuals of a given marine species would grad- 

 ually accustom themselves to the less saline water at the mouth 

 of the river, and penetrating by degrees further up the latter, 

 would develop into freshwater forms more or less structurally 

 distinct from the ancestral type. Thus the Stickostemma 

 eilhardi of the neighborhood of Berlin, represents probably an 

 originally marine species which has penetrated from the North 

 Sea up the River Spree ; and a similar migration up other 

 rivers has probably led to the development of most nemerteans 

 found in rivers. Other forms, however, as has been elucidated 

 by Du Plessis,^ represent species of a fauna relicta, in that an 

 inland sea which was formerly saline, has gradually become 

 freshwater, producing in this way freshwater forms. Such has 

 probably been the origin of the nemerteans in the Swiss lakes, 

 and in similar inland seas. Accordingly, freshwater nemerteans 

 have been produced from marine species, (i) by either an active 

 migration up into bodies of fresh water, or (2) by remaining in 

 the same locality while their environment itself has changed ; 

 in other words, we might speak of an active or 2, passive adapta- 

 tion, both of which might lead to the same end. 



As to the derivation of the land nemerteans, two possible 

 sources are present: (i) either they are derived from marine 

 forms, or (2) from freshwater forms. On the ground of the 

 first possibility, we must suppose that they had accustomed 



1 Sur I'origine et la reijartition des Turbellaries de la Faune profonde du lac 

 Leman. Actes Soc. Helv. 60, 1878. 



