504 Transactions. 
and their distribution in the main supports this view. Thus bright-green 
algae are usually highest in the series, dark-green and brown algae 
predominate about low-tide mark, whilst red algae are most plentiful 
below this. The red pigment is presumed to check the harmful action of 
the actinic violet rays, which are proportionately stronger at a little depth 
than at the surface. But there are many exceptions to the order in 
which the classes of algae usually occur. Thus Bostrychia arbuscula and 
Caulacanthus spinellus, which occur highest in the integtidal belt, belong 
to the Rhodophyceae, while Chaetomorpha and Caulerpa are bright-green 
algae found amongst red algae at low-tide mark. 
There appears to be a relation similar to that observed in algae between 
not unlike that of the Rhodophyceae. Under stones the tests are colour- 
less, with a pink or reddish tinge (Corella), or distinctly blue (Styela 
caerulea). 
EVAPORATION. 
(a.) Shelled Animals—Undoubtedly the main character by virtue of 
which animals are able to conserve moisture during the retirement of the 
tide is an external shell. The shell, by reason of its usually great strength, 
organ which secretes ‘the shell, mainly by its edge; but additions to the 
thickness of the shell are made internally by the portion of the mantle 
