SEX DETERMINATION IN DINOPHILUS GYROCILIATUS. 337 
and are probably one and the same. The form on which the 
following work has been done is one of these three species, 
though exactly which I have been unable to decide. I have 
placed it under the head of D. gyrociliatus, as this is the 
oldest of these names. Figures of the male and female, 
drawn to scale, are shown in Text-tig. 1. It will be seen 
that the female is very much larger than the male, and that 
the sexual dimorphism is well marked. 
The species D. gyrociliatus was established by Schmidt 
( 15 ) from material obtained from the old harbour at Naples 
and described by him in 1857. He gave a short description, 
with one figure, from which it is not possible to gain very 
much information. It was subsequently studied by Meyer ( 9 ), 
who gives a figure of it in his studies on the embryology of 
Annelids. At about the same time it was made the subject 
of a lengthy paper by Repiachoff ( 13 ), who described its 
anatomy and who first found the male form. It now appears 
through the great extension of the harbour at Naples, and 
the consequent contamination of the sea-water, to have 
forsaken its old habitat in the Porto Vecchia, for during* 
the last ten years I have repeatedly looked for it during 
different visits to Naples, without being able to find it. It 
was formerly found in abundance on the masses of Bugula 
attached to the piers of the old harbour. Recently I have 
often examined the Bugula growing there, without however 
ever in any case observing Dinophilus. 
I am unable to say, therefore, from personal experience, if 
the species obtained at Plymouth, and on which the present 
work has been done, is the same as D. gyrociliatus 
of Naples. Prof. Meyer, of Kasan, however, has kindly 
examined some material that I have sent him, and he tells 
me that he find it very similar, as far as he can determine 
from preserved material, to D. gyrociliatus, with which 
he was formerly familiar at Naples. 
The other species with which it may possibly be identified 
is, first, I), apatris, described by Korsclielt in 1882, from a 
salt-water aquarium at Freibourg. It has been suggested 
