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



similarity of structure; but such evidence can by no means be employed in system- 

 atic zoology until we know more about the essential nature of euryhalinism, regard- 

 ing which at present we are profoundby ignorant. These experiments on Manayunkia 

 and Fabricia indicate that euryhalinism is a quality of living animals of the most 

 persistent and fundamental sort, and that it is not lost (as shown by the experi- 

 ments with Nereis limnicola and with Manayunkia) by very prolonged residence in 

 fresh water. Nor can we predicate that it does not exist in a marine form that is 

 nowhere found living in fresh or brackish water. 



On a 'priori grounds we might infer that forms of small size and comparatively 

 simple structure would most likely be able to endure the change in passing from sea 

 water to fresh water. Forms that obtain their food by creating a vortex through 

 ciliary action, and are able to feed omnivorously upon microscopic organisms, would 

 also seem to have a distinct advantage. The various dwarfish and apparently degen- 

 erate fresh-water and brackish-water Sabellidse are examples which seemingly justify 

 this inference; but such rules break down completely when we attempt to apply 

 them to the Nereidae. Many of the euryhaline members of this family, it is true, are 

 smaller than the average of their exclusively marine congeners; but not all of them 

 are. Lycastis ouananyensis is stated by Gravier (:01) to be from 120 to 200 milli- 

 metres in length and 7 millimetres in breadth. L. hawaiiensis is also a worm of 

 goodly dimensions. The fresh- or brackish-water habitats of such forms as Nereis 

 limnicola and N. virens may quite as reasonably be regarded as the cause of their 

 small size as in any measure the result of it. Such habitats no doubt often entail a 

 diminished food-supply, and nearly always a much less severe struggle for existence. 

 We note in nearly all fresh-water invertebrates puny size and lack of vigor, as com- 

 pared with marine invertebrates of the same groups. 



The possibility of a marine animal establishing itself in fresh water is deter- 

 mined by four essential factors, two of which are intrinsic, and two extrinsic : 



1. The possession of euryhalinism, enabling it to endure the diminished spe- 

 cific gravity of its medium. 



2. Presence of a suitable fresh-water habitat, accessible from the sea, with inter- 

 mediate brackish-water stations. 



3. Possibility of obtaining food in the new habitat. 



4. The capability of breeding in the new environment. 



The degree of saltness of the water to which euryhaline Polychaeta can accom- 

 modate themselves, often exceeds that of the ocean. Such forms occur in salines. 

 As Ferronniere (:01) has pointed out, there is a similarity between the faunas of 

 salines and of brackish water. 



