REPORT OF THE DIRECTOR 1920-21 157 



Comparative number of species of invertebrates in the Baltic, etc. 



Phyla 



Waters 



around 



Great 



Britain 



Baltic 

 as a 

 whole 



Bay 



of 



Kiel 



Bay of 

 Trave- 

 rnunde 





35 permille 



78 permille 



20 permille 



12 permille 





69 

 42 

 98 

 48 



IOI 











7 

 28 



6 

 68 

 11 

 SO 

 68 



5 



3 

 24 



5 

 SO 



8 

 36 

 64 



4 







8 









26 





s 











682 



40 





4 









Total 



1040 



243 



194 



107 







Another striking change has been noted in the character of the 

 Baltic fauna which may likewise be correlated with the variation 

 in salinity. As the stenohaline forms disappear entirely, euryhaline 

 forms become dwarfed. Mobius (1873, p. 138) reports dwarfing 

 of worms, and of a copepod, very noticeable even in the short 

 distance from Arendal (Norway coast, on Skager Rak) to Kiel. The 

 dwarfing of fishes has also been noted {see O'Connell, p. 72). The 

 animals of the eastern basin are more dwarfed than in the western 

 basin, and the best examples are found among the mollusks, in 

 which group in addition to being dwarfed, the shells become poor 

 in lime. 



Mytilus edulis at Kiel attains a length of 8-9 cm ; in 

 the eastern basin (for example at Stolpe bank, Gotland and Dalaro) 

 this mollusk reaches a length of only 3-4 cm. In the clayey mud 

 of the sea bottom in various places in the eastern basin are found 

 very many conchiolin coverings of Mytilus edulis and 

 Macoma balthica (groenlandica); often the two 

 brown conchiolin coverings are still bound together by the ligament 

 at the back in complete shell form. This occurrence is readily 

 explained. It has been found in the case of these two species, 

 in the eastern basin, that the lime layers of the shell are extraordi- 

 narily thin, and therefore so brittle that they can be easily crushed 

 between the fingers. Because of its thinness, the lime layer of the 

 shells is very soon dissolved after the death of the animal (Mobius 

 1873, p. 138). 



A very noteworthy case of dwarfing is exemplified byCardium 

 e d u 1 e, the common European cockle, which has a large, rough, 

 thick shell and thrives best under purely marine conditions. This 



