OPISTHOBEANCHIATA FROM THE ABROLHOS ISLANDS. 525 
discarded. It is obvious that, if this sort of procedure goes on unchecked, the 
ultimate result will be a chaotic muddle, out of which it will be very difficult 
to find a way. The present author, in conjunction with Iredale (51a), has 
spent a great deal of time in an attempt to straighten out the tangle of the 
synonymy of the British Nudibranchiata, and, as a result, certain fairly 
widely used generic names have been replaced by less known ones. The very 
obvious criticism will be made that such changes will cause a certain amount 
of inconvenience, and it must be admitted at the outset that tliis is so. On 
the other hand, a few hours' work in the taxonomy of any group will 
soon show that this inconvenience is nothing to the deadlocks that arise 
from the slipshod and incorrect use of names. The common remark that 
"everyone knows what so-and-so means" is not merely untrue, for the 
original describer of the species would not know, but it is frequently a flimsy 
excuse for avoiding the work necessary to justify the em^Dloyment of a 
correct terminology. Furthermore, it becomes impossible to discuss such 
topics as the comparative anatomy, systematic relationship, and distribution 
of the members of a large group of animals unless one can be more or less 
certain of the species involved. Wherever changes from the common 
iisage have been made, the reasons for so doing have been briefly given and a 
synonymy provided. 
My friend Professor Dakin handed the collection over to me for exami- 
nation, and I wish to offer him my sincere thanks not only for so doing but 
also for giving the colour-notes mentioned above. I also desire to thank the 
authorities of the British Museum — in particular, Mr. G. 0. Robson — for the 
privilege of working at the Museum, where ready reference to books and 
specimens greatly facilitated my work ; and, lastly, also Professor Dendy, 
F.R.S., of King's College, London, in whose laboratories the dissections, 
preparations of the radulse, and the drawings were made. 
All the drawings of teeth were made with the camera lucida from my own 
preparations. Type-specimens, examples of all the species, and, when 
present, the radulae have been deposited in the British Museum. 
Order OPISTHOBEANCHIATA H. Milne-Edwards, 1848. 
The Gastropoda belonging to the class Euthyneura are all hermaphrodite 
and characterised by the detorsion of their visceral mass and nerve- 
commissure; in addition, they generally have two pairs of tentacles and 
exhibit a tendency, sometimes marked, to concentration of the nervous 
system. They fall fairly naturally into two orders — the Pulmonata, which 
are adapted for aerial respiration and life on land, and the Opisthobranchiata 
which are adapted for aquatic respiration and a marine life. The 
Opisthobranchs thus come to form a well-defined group ; on the other hand 
however, the division of this order is not quite so simple. 
