OPENING ADDRESS. 19 
are always conspicuous so far as I have noticed, even in 
their natural condition. Then again the species of Holis 
are rarely found hiding in or on other animals, they are 
not shy, and they are active in their habits—altogether 
they seem rather to court observation than to shun it. 
When we remember that the species of Holis are protected 
by the numerous stinging cells in the cnidophorous sacs 
placed on the tips of all the dorsal cerata, and that they do 
not seem to be eaten by other animals, we have at once 
an explanation of their fearless habits and of their con- 
Spicuous appearance. The brilliant colours are in this 
case of a warning nature, for the purpose of rendering the 
animal provided with the stinging cells noticeable and 
recognisable. 
Upon such cases as these then I base my view that the 
chief function of the cerata of these Nudibranchs is by 
varied shapes and colours to enable the animal to assume 
appearances which are in some cases protective and in 
other cases conspicuous and warning, according as may be 
found best suited to the surroundings and mode of life, 
and in this it seems to me we have an explanation of the 
extraordinary development of these otherwise mysterious 
processes of the body wall. If this is the correct inter- 
pretation, it affords us marked examples of real utility in 
specific characters which may at first sight appear to 
depend upon trivial differences in form and colouring. 
TUNICATA. 
Turning now to the Tunicata, we come to a group at 
which I have been working for a much longer time, and in 
which I found it necessary to set to and re-determine 
for myself the true diagnostic characters of many of the 
species and genera. Most of the older descriptions of the 
species of Tunicata consisted merely of a short account 
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