40 LIVERPOOL BIOLOGICAL SOCIETY. 
reefs of this gregarious rock-building Annelid have a 
curious external resemblance—superficial only, as in 
structure and mode of formation they are of course utterly 
different—to the forms produced by coral masses in 
lagoons and amongst coral reefs and islands. It might 
be possible by a continuous study, on the spot, of this 
Sabellaria at Hilbre to determine what part the various 
factors—food, currents, muddy water, presence of sand 
and rock, exposure to waves, and the arrangement of the 
animals in the mass—take in producing the various shapes 
and in promoting or retarding growth. 
Another interesting piece of work which could be carried 
out at Hilbre, or at Puffin Island, would be to determine 
the various heights above low water mark at which the 
different kinds of marine animals and plants are able to 
exist. Many of the Zoophytes and other animals living 
about half-tide are found to be in a very dry and shrivelled 
up condition before the tide reaches them, but they revive 
and expand instantly on being placed in water. I have 
found the Polyzoon Flustrella hispida, at Hilbre Island, 
in a living and healthy condition only about a yard below 
high water mark. In such a position the animal must 
be exposed to the air during about five-sixths of its 
existence, and since its tentacles can only be expanded 
when covered with water the entire food supply must be 
taken in during the remaining one-sixth, at and about the 
time of high tide. The short opportunity of feeding will 
therefore only occur twice in each twenty-four hours, and 
during the long intervals the animal must remain in a 
contracted and possibly a somewhat dormant condition. 
It would be important to determine the precise nature of 
the food of Flustrella hispida at Hilbre, and whether it 
varies with different tides, winds and seasons; and an 
interesting series of experiments might be made by some 
