SEA-FISHERIES LABORATORY. 135 



they are the only forms able to withstand the severe conditions 

 prevailing, in a degree sufficient to ensure the maintenance of 

 the species. It cannot be depth that keeps Macoma species 

 out of the Venus area. My a armaria, Macoma balthica, 

 C. edule and Hydrobia live in 20 m. in the Baltic, but in such 

 places there are no Echinoderms." 



Within the Macoma community in the beds examined, 

 I find considerable variation in the distribution of the species 

 inhabiting it. The chief of these are Arenicola marina, Cardium 

 edule, Macoma balthica, Tellina tenuis and Paludestrina stagnalis, 

 while, in addition, there are a number of others of lesser signi- 

 ficance which will be mentioned later. 



Macoma has a wider range than Cardium, occurring high 

 in tides and in other places where Cardium is absent, and the 

 same can be said for Arenicola. 



We are also dealing with a stratification of species which 

 explains some of the peculiarities of distribution : typically 

 these strata are : — ■ 



1. The floating and swimming animals (Plankton) Crangon, 



My sis and other Crustacea and fish and their larvae, 

 that come and go with the tide ; some, however, 

 remaining in pools and streams left during low water. 



2. Surface stratum, with Paludestrina dominant. 



3. Stratum of Cardium edule one to one and half inches 



deep, short siphons protruded to the surface. 



4. Macoma balthica (or Tellina tenuis) stratum, one and 



half to three inches below surface ; siphons long. 



5. Stratum of Arenicola marina, six inches to two feet 



below surface, burrows formed. 

 Cardium requires a moderately firm bed, especially in the 

 more exposed situations, and further, the amount of organic 

 matter present is, in my opinion, probably correlated with the 

 relative abundance of the species. Macoma is less affected by 

 surface conditions, and Arenicola considerably less. 



