 |
|
It
is not only heat regulation that is important for survival, waves and predators
are also of interest. Therefore, it is not always the large round white
organisms that survive and the most common on the beach.
An effective way of losing heat is to let water evaporate,
but this also can dehydrate the organism. Certain organisms collect so much
water as possible between their shell and body before they are left stranded
at low tide. Barnacles
do this, so they are able to give off heat by means of evaporation.
To breath air
Most animals that inhabit the shoreline have
gills that enable them to breath in water. Gills are outgrowths of the body
that are heavily creased, thin walled and greatly branched. This gives them
a very large area for an effective gas exchange between the body and the
surrounding water, but this construction can also result in dehydration
if they come in contact with the air. For this reason, most beach organisms
have their respiratory organs contained within a body cavity. Here, the
gills are well protected from the environment and their surfaces are easily
kept moist without too great a water loss. With mussels and shellfish, the
protective cavity is composed of a skin like casing known as the mantle.
Outside the mantle is the shell that gives further protection. Barnacles
also have a type of mantle and it is the mantle wall which is furnished
with blood vessels and functions as the respiratory organ.
To conserve water at low tide, certain mussels, shellfish
and barnacles must enclose themselves totally within their shells. At such
times they can enter a trance
state, where they do not even need oxygen from the surrounding water.
 |
Page 27 of
52 |
 |
|
