478 DEPTHS OF THE OCEAN 



and Echimts acutus (forma flemingi) is curious. The former is 

 very common out among the skerries, while E. aczUus confines 

 itself to a few localities, but on ascending the fjords E. escidentiLs 

 becomes scarcer, and descends to greater depths, whereas 

 E. acutus occurs in the greatest abundance. A similar distribu- 

 tion characterises the sea-urchins Pai^echinus miliaris and 

 Strongylocentrotus drobacJiiensis, which much resemble one 

 another in outward appearance, and are both exceedingly 

 plentiful in their different localities. Strongylocentrotus lives in 

 the more open estuaries and bays of the skerries, whereas 

 Parechinus miliaris keeps to sheltered waters, and especially to 

 pools. For instance, in a pool south of Bergen (the Inderoe 

 Poll) I found Pai'echimis miliaris literally in thousands, but there 

 was not a single specimen of Strongylocentrotus ; in the neigh- 

 bourhood of Bergen again I collected from another pool of a 

 rather less typical character, sixty-three specimens oi Pai^echinus, 

 and only three specimens of Strongylocentrottis. This difference 

 has not been explained, though most probably the cause is to be 

 found in the difference in temperature. Pools contain water 

 of a much higher temperature than the sea outside, and most 

 likely Pai^eckinus miliaris requires for its reproduction warmer 

 water than Stro7igylocentrotus. It is interesting to note that, 

 according to Petersen, there is the same diversity between these 

 two forms in the Kattegat. 



The foregoing is not meant to be even an approximately 

 complete account of the forms inhabiting the skerries and the 

 fjords, my sole object having been to show that the dissimilarity 

 in physical conditions (temperature, salinity, etc.) and in the 

 nature of the bottom, between the skerries and the inner parts 

 of the fjords, determines the difference in their biological 

 conditions. 



Those areas of the littoral zone which have been called 

 Pools, pools, or "polls" (see p. 225), are salt water basins connected 

 with the sea outside by a shallow channel. The pools vary in 

 depth, the deepest not exceeding 30 metres. One feature 

 which they all have in common is that their channels to the sea 

 are far shallower than their basins. The surface is always 

 covered by a layer of more or less fresh water derived from the 

 land, having a lower temperature than the salt-water layer 

 underneath. About i^ or 2 metres below the surface the 

 temperature in some summers may rise to 30° C. or even more, 

 while that of the surface-layer does not rise above 18° or 20° C, 



