i] THE HABITAT OF CONVOLUTA 17 



seen, by the light of a lantern, lying in the river-films 

 of their diurnal stations. 



Except for a rich micro-flora and -fauna of diatoms, 

 bacteria and infusoria, except for a rare, solitary 

 enemy another worm, a species of Plagiostoma, which 

 shovels live Convolutas by the hundred into its capa- 

 cious body except for an occasional, small shore-crab, 

 picking its way with rolling but deliberate gait over 

 the patches, C. roscoffensis enjoys undisputed posses- 

 sion of its tract of foreshore. Though the wastage 

 from each colony must be prodigious, every incoming 

 tide taking toll, yet the species, fecund and resource- 

 ful, rises superior to the circumstances of its environ- 

 ment and maintains itself in the strange situation 

 which fate has chosen for it. 



The roscoffensis zone (Figs. 1 and 7) is as localised 

 as the range of distribution of the species is restricted. 

 The upper limit of the zone is marked by the level 

 reached by high water at the slackest of the neap tides : 

 for, further landward, C. roscoffensis could not obtain 

 at all tidal periods the diurnal plunge-bath without 

 which it does not thrive. Risk of desiccation bars 

 its more landward advance. The lower limit of the 

 zone is but a few yards seaward, for C. roscoffensis 

 loves the light and ensues it. 



At every making tide, this zone is submerged and 

 C. roscoffensis becomes a submarine plant-animal 

 sheltering beneath the surface of the sand out of 



K. 2 



