OPENING ADDRESS. 17 
points, whether of external appearance or of internal 
anatomy, which sufficiently distinguish allied species from 
one another. 
Evidently there is still a great deal of critical work to 
be done in the examination and diagnosis of the various 
species of Nudibranchs, but there can be little doubt that 
in at any rate very many of the species, characters derived 
from the external appearance as given by Alder and 
Hancock will figure amongst the revised specific descrip- 
tions, and the question then arises, Can we account for 
_ the varied shapes and colours of the curious horn-like and 
often dendritic projections from the bodies of Nudibranchs 
on the ground of utility? And I believe we can, as the 
following few instances will show you. 
In Tritonia (or Candiella) plebeia, in addition to the 
rhinophores or smelling tentacles near the anterior end of 
the animal, we find a series of small branched projections 
called ‘‘cerata’”’ placed along each side of the back. These 
are not branchie, but are merely outgrowths from the 
body wall. This species is fairly abundant at Puffin Island 
and at Hilbre Island, and is found in those localities 
creeping over the surface of colonies of Alcyonium digi- 
tatum. The specimens of Tritona are marked with many 
colours, including tints of yellow, brown, blue, grey, black 
and opaque white; and when examined in a vessel by 
themselves considerable differences between individuals 
are noticed, but when in their natural condition on the 
Alcyonium colony they are nearly all equally inconspicuous. 
The colonies of Alcyoniwm differ considerably amongst 
themselves in tint, some being whiter, others greyer, and 
others yellower than the rest; different parts of the same 
colony also vary in appearance on account of the different 
states of expansion of the polypes, and on account of 
irregularities of the surface, and of adhering sand and 
2 
