The body, however, is always extremely slender with a diameter of not more than 500 μ, thus enabling the animals to move through the interstitial system without actively replacing the sand grains; all the listed species, therefore, are considered to be mesopsammon forms sensu Broaden (1962)
Source: wiktionary
The mesopsammon of intertidal environments and the many physical, chemical and mineralogical factors which affect their presence are very complex.
Source: wiktionary
As is the case with mesopsammic representatives of other major taxa, interstitial nemertines constitute a unique biological grouping recognizable by specializations that are characteristic of mesopsammon (see Swedmark, 1964, for review of the nature of interstitial organisms).
Source: wiktionary