506 Transactions. 
edge of the shell. Water is retained within the branchial chamber precisely 
as in limpets. 
Brachiopoda.—The lamp shells possess two shelly valves placed dorsally 
and ventrally, the animal being attached to the rock by a muse 
peduncle passing through an aperture or notch in the ventral valve. These 
operculum is not usually used for the purpose of retaining water, this being - 
done by the animal applying the peristome of the shell to the rock-surface. 
If, however, the shell gets displaced and the water flows away, the anim 
may seek a moist spot, or else draw in behind its operculum until the - 
tide overtakes it. Coming now to low-tide mark, we find on rocky coasts 
that the operculum is usually normally developed in the spiral shells. 
Here, perhaps, life is strenuous and protection needed ; but on sand-flats 
the operculum becomes reduced so that it can no longer close the aper 
chamber on each side, outside the attachment of the legs. In the en x 
spaces so formed are situated the gills, to which water enters only by narrow 
slits at the bases of the legs, and so in these nearly closed chambers the 
gills are kept quite moist while the crab is running about the rocks. 4 
-) Cor ests. — Many animals living near low-tide mark, 9n t 
_ Consequently exposed for short periods each day, do not secrete shells, bur 
