OPISTHOBRANCHIATA PROJf THE ABROLHOS ISLANDS. 521 
Report on Opisthobranchiata from the Abrolhos Islands, Western Australia, 
with Description of a new parasitic Copepod. By Ghas. H. O'Donoghue, 
D.Sc, F.R.S.C, Professor of Zoology, University of Manitoba, Canada. 
(Oomimmicated by Prof. W. J. Dakin, D.Sc, F.L.S.) 
[Percy Sladen Trust Expedition to the Abrolhos Islands 
under the leadership of Prof. AV. J. Dakin.] 
(Plates 27-30.) 
[Read 21st June, 192.3.] 
From a very cursory examination of the distribution of the Opisthobranchs, 
it is evident that the Indian Ocean is very rich in members of this group and 
is characterized particularly by certain genera. For this reason we may 
look forward to the coasts of Australia furnishing a large number of 
interesting forms. Up to the present, however, they have not been studied 
in this area with any degree of thoroughness, save in the neighbourhood of 
Sydney by Angas (7) and less extensively in South Australia by Basedow 
and Hedley (8). In Western Australia it may be said that no systematic 
collecting has been done, so that our knowledge of the Opisthobranch fauna 
of this coast is very limited. 
The present collection, while small in numbers, is, nevertheless, interesting 
and, it is to be hoped, useful. Certain of the species themselves appear to be 
new, and some of the others, although previously recorded, have only been 
incompletely described, so that it has been possible in all cases to give here 
further essential information regarding their structure or distribution and to 
clear up certain doubtful points. In these ways therefore it will extend our 
hitherto meagre knowledge of AVestern Australian forms and so help to fill 
a noticeable gap. 
The student of the Opisthobranchiata, and especially the Nudibrancbiata, 
is generally faced with one of two very serious difficulties. On the one 
hand, if he collects living material, he does not as a rule have the oppor- 
tunity to dissect it and, more particularly, to examine the radulse. On the 
other hand, if he is Vv-orking out a collection in a museum or laboratory, 
while he is able to make such dissections and preparations of the radulse as 
the material allows, he is usually not in a position to recognize the original 
colour, or perhaps even shape, of the living creature. I doubt if any other 
group of animals is so disappointing when preserved ; anyone used to seeing 
them alive, with their beautiful coloration and often graceful shape, would 
hardly recognize them in the shrivelled discoloured lumps that they become 
after preservation. Hence it follows that unless the two aspects are dealt 
