84 Prof. K. Mobius on the 



This Gasteropod becomes developed in the slightly salt water 

 near Gotland to the same size as in more than normally salt 

 lakes on the shore of the North Sea. 



Because the Baltic animals are eurythermal and euryhaline 

 they are capable of living both at small and great depths and 

 of maintaining their ground throughout long geological periods. 



Among the animals catalogued there is only one true 

 brackish-water animal, namely Cordylophora lacustris, a 

 polype which lives only in very slightly salt water, and perishes 

 both in fresh water and in water containing a larger amount 

 of salt*. 



Besides this brackish- water animal and the euryhaline ani- 

 mals, a number of freshwater animals live in the eastern basin. 

 These are such as can bear slightly salt water. Nature has 

 not succeeded in habituating them to the larger amounts of 

 salt in the western basin, although probably she makes fresh 

 attempts every year to diffuse freshwater animals from the 

 mouths of rivers and brackish-water bays further into the sea. 

 The pioneers constantly sent out, however, have been unable 

 to force their way into the Salter region. Such miscarriages 

 of Nature in her constant forward march must render us very 

 cautious in estimating the value of experiments made in 

 aquaria for the purpose of habituating freshwater animals to 

 salt water and marine animals to fresh water. I refer here 

 especially to the recent experiments of Plateau upon Asellus 

 aqnaticus and some other animalsf. 



The number of species diminishes suddenly when we pass 

 from the shallow and more saline western basin into the deeper 

 and less saline eastern basin. We found most of the species 

 of the latter at depths from 0-20 fathoms ; they became 

 fewer at 20-50 fathoms, and very few from 50-95 fathoms. 



The following animals are inhabitants of the greater depths 

 of the eastern basin : — 



Astarte borealis down to 46 fathoms. 



Tellina halthica „ 49 „ 



Cuma Rathkei „ 49 „ 



Idotea entomon „ 60 „ 



Astemma rujifrons „ 50 „ 



Nemertes gesserensis „ 60 „ 



Halicryptus sjnnulosus .... „ 50 „ 



Scoloplos armiger „ 46 „ 



Terebellides Stromii „ 47 „ 



Polynoe cirrata ., 95 „ 



* See p. 100 of the Report, and also E. Schultze, ' Bau und Entwick- 

 lung von Cordylophora lacu&trisf 1871, pp. 43-48. 



+ Mem. Acad. Belg. 1870, and Ann. & Mag. Nat. Hist. ser. 4. vol. vii. 

 1871, p. 302. 



