Invertebrate Fauna of the Baltic. 83 



By far the greater number of the Invertebrata of the Baltic 

 are also inhabitants of the North Atlantic Ocean. Of many 

 of them we know that they are spread into the icy polar sea, 

 and as far as the African coast. With regard to the shell- 

 bearing mollusca, this was demonstrated in detail in the 

 ' Fauna der Kieler Bucht.' This wide distribution of the 

 Baltic animals, their ability to live in warm, temperate, and 

 cold seas, becomes intelligible when we have made ourselves 

 acquainted with the temperatures which they have to endure 

 in the Baltic. In the physico-chemical section of this Report 

 it is shown by a table (xxxii.), founded upon three years' 

 observations by Dr. H. A. Meyer, that the differences of 

 temperature in the superficial layer rose to 14 o, 9-20° 

 (=26°'8-36° F.), attained 13°-3-17°'3 ( = 23°-9-31°-14 F.) 

 at 5 fathoms, and even at a depth of 16 fathoms still amounted 

 to 9°-2-12°'2 ( = 16°-56-21°-96 F.). In all the strata of the 

 water, even in the deepest, at the coldest season, the animals 

 of the Baltic have to endure a temperature which sinks to the 

 freezing-point of salt water, therefore below zero ( = 32° F.). 

 In summer and autumn, on the contrary, they are exposed to 

 a pretty high temperature. The different temperatures which 

 the individuals of a species experience in the course of a year 

 in the Baltic are undergone at the same time by other indi- 

 viduals of the same species which live in the Mediterranean, 

 the North Sea, and the north polar sea. The Baltic contains 

 only a selection of such Atlantic and Polar animals as are 

 capable of supporting great differences of temperature. For 

 this reason they may be called eury thermal* animals, in con- 

 tradistinction to those animals which thrive only in warm or 

 cold and tolerably constant temperatures, such as the tropical 

 and exclusively arctic marine animals, both of which may on 

 this account be denominated stenothermal^ animals. 



All the marine animals of the Baltic have further the faculty 

 of living in sea-water containing a variable amount of salt ; 

 those Baltic animals which also occur in the Mediterranean 

 can bear a larger amount of salt than the Atlantic ocean con- 

 tains. This faculty of the Baltic animals is by no means 

 indicated by calling them brackish-water animals ; on the con- 

 trary, this expression carries our thoughts away from one of 

 their most remarkable peculiarities ; for animals which can live 

 not only in slightly but also in strongly salt water are not 

 brackish-water, but euryhaline\ animals. 



A very perfectly euryhaline animal is Hydrobia ulvw. 



* From evpvs, wide, and deppos, heat, 

 t From arrevos, narrow, and 6eppos, heat. 

 X From evpvs, wide, and SXs, salt. 



6* 



