INVERTEBRATE BOTTOM FAUNA 477 



(2) The Fjords. — We have seen that the fauna of the Littoral zone. 

 Httoral zone among the skerries, especially in the tidal area and 

 laminaria belt, is abundant both in species and individuals. 

 There is a diminution, however, as we penetrate farther into 

 the fjords. In the tidal area of the inner fjords, and at greater 

 depths also, we miss the limpet and the purple snail, while the 

 hydroids to be found on the fucus in the skerries become less 

 and less abundant, until even Dyna77iena piunila disappears/ 

 This change in the fauna is mainly due to the decrease in 

 salinity, since the surface of the inner fjords, for a great part of 

 the year at any rate, is occupied by a layer of less saline water 

 in which these forms cannot thrive. Far up the fjords, however, 

 in the tidal area, we get the barnacle, the mussel Mytiliis, and the 

 black periwinkle, which seem to be less affected by a difference 

 in salinity, though even they require a certain percentage of salt, 

 since they disappear, for instance, from the tidal area in the more 

 enclosed parts of the fjords, where, owing to the great accession 

 of fresh water, the salinity is particularly low. The mussel and 

 black periwinkle, it is true, may sometimes occur even here 

 also, but only in fairly deep water. We also find the horse 

 mussel in the fjords. The great thickets of Laniinaria hyper- 

 borea, which are so characteristic of the skerries, are absent 

 from the inner fjords, and so are most of the forms associated 

 with them. In their place, however, we get Laminaria digitata 

 and L. sacckarina, but in comparatively small quantities. 



The difference between the inner fjords and the skerries is 

 not so marked when we descend to greater depths, since a 

 good many forms are equally at home in both. Some of the 

 littoral fauna, like the lancelet, appear to avoid the fjords 

 altogether.^ Two forms, which rarely ascend far up the fjords 

 of West Norway, are the lobster and the common edible crab ; 

 but the common shore crab {Carcimts moenas) penetrates to 

 their inmost recesses. The big black sea-slug i^Cucumaria 

 frondosa) is another form which abounds among the skerries 

 and in the outer parts of the fjords, but very exceptionally 

 penetrates far in. No doubt their absence is due to the feeble 

 currents, or the greater or less accessions of fresh water 

 prevailing in the fjords — local conditions that are bound to 

 affect the distribution of the fauna. 



The distribution of the two sea-urchins Echinus escnlcnttts 



1 It is interesting to note that Dynamena piimila is also found in the estuary of the Elbe as 

 far up as Cuxhaven. 



^ The reason for this may perhaps be that the lancelet requires pure sand or shell-sand to 

 live in, while the bottom of the fjords generally consists of mud. 



