INTRODUCTION. 9 



zone at Hilbre into a series of regions or sub-zones, separated 

 by contour lines parallel with low water mark, and to investi- 

 gate the fauna and flora of each region separately, so as to 

 determine their characteristic animals and seaweeds, and the 

 relative capacities the diff'erent species possess for with- 

 standing exposure to air and sunshine. Flustrella hispida 

 was found, last summer, attached to the rock, within about 

 one yard of high water mark, in a living and healthy condition. 

 From its position, this animal can only be immersed in water 

 during a small proportion of its life, at and about the time of 

 high tide. It will be interesting to discover whether it shares 

 this condition with other marine animals and to determine 

 the nature of the food in such cases, and whether the species 

 is able to stand considerable variation in the amount of its 

 periodic exposure to air. 



On some parts of the Hilbre shore, especially at the 

 northern end where sand and rock meet, a gregarious tubi- 

 colous annelid, Sahellaria alveolata, is present in great 

 abundance, and produces, by building up tubes formed of 

 sand-grains, a loose, porous, but crisp and brittle, mass, which 

 crumbles when walked upon, but which is constantly being 

 renewed, and has its injuries repaired by the living annelid 

 within. This, from its abundance and thickness, must have 

 a very considerable effect in protecting the shore from the 

 erosive action of the sea. The masses, hummocks, pla- 

 teaux, ledges, and small reefs of this rock-building annelid, 

 have often a curious external resemblance, superficial only, 

 of course, to the forms produced by coral masses amongst 

 coral reefs and islands. It might be possible, by a con- 

 tinuous study on the spot of this Sahellaria at Hilbre, to 

 determine what part the various factors — food, currents, 

 muddy water, presence of sand and rock, exposure to waves, 

 and arrangement of animals in the mass — take in producing 

 the different shapes, and in favouring and retarding growth. 



