Invertebrate Fauna of the Baltic. 85 



Thus the Crustacea and Vermes go deepest. Bivalve mol- 

 lusca were no longer found in the great depths, although organic 

 substances still existed there, as is shown by Dr. Behrens 

 from his investigations of samples from the bottom. As the 

 bivalve mollusca are among those important animals which 

 are capable of converting dead organic substances of the sea- 

 bottom into living animal matter, the number of carnivorous 

 animals must also be diminished where they are wanting, 

 unless other mud-eaters carry out the business of the first 

 preparation of flesh in their stead. 



Our knowledge of the physico-chemical conditions of the 

 greater depths is not sufficient to explain satisfactorily the 

 disappearance of the animals. Besides the small amount of 

 salt and the persistently low temperature, one of the causes of 

 the impoverishment of the fauna in the greater depths of the 

 eastern basin of the Baltic must be sought in the weakening 

 of the currents which assist the change of gases and carry 

 food with them. 



The ten species which were found at depths of 46-95 

 fathoms are all inhabitants of higher regions. In general, the 

 animals of the eastern basin of the Baltic, as may be seen 

 from the list of their localities, accommodate themselves to 

 various depths and to the most various conditions of the 

 bottom. They possess a greater capacity of adaptation to 

 differences in the amount of salt, in temperature, depth, and 

 bottom, than those species which occur only in the western 

 basin. This very pliable nature has given them the predomi- 

 nance over the whole eastern region ; and here, therefore, they 

 can develop into enormous multitudes of individuals without 

 having to maintain a contest for their dwelling-place and 

 nourishment with fresh immigrants from the west. 



The species which occur in particular abundance are the 

 following : — Hydrobia ulvce^ Mytilus edulis, Tellina balthica, 

 Cardium edule, Palaimon squilla, Guma Rathkei, Mysis spinu- 

 losa, M. vulgaris, Gammarus locusta, Pontoporeia femorata, 

 Idotea entomon, I. tricuspidata, Jaira marina, Temora longi- 

 cornis, Polynoe cirrata, Scoloplos armiger, Nephthys ciliata } 

 Nereis diversicolor, Terebellides Stromii, Halicryptus spinu- 

 losus, Membranipora pilosa } Alcyonidium mytili, and Medusa 

 aurita. 



The simultaneous production of many individuals of the 

 same species at the same place is of importance for the nutri- 

 tion of the edible fishes. As soon as these have found the 

 dwelling-place of a great multitude of individuals of bivalve 

 mollusca, worms, Crustacea, or other eatable animals, they can 

 feed themselves with ease. This also explains why we often 

 find in the stomachs of fishes many animals of the same species. 



