158 Prof. M'Intosh’s Notes from the 
warm Mediterranean waters. Environment would thus 
appear to be a factor of importance in some instances, if not 
in all. Moreover, it would seem to be as unnecessary to 
place the weight given by some authors on the presence or 
absence of an operculum as to separate like species of oxen 
by the presence or absence of horns. Some, like Claparéde 
and Fauvel, perhaps, might be disposed to separate as 
distinct species those with and those without an operculum, 
or to regard either as a sudden and an important mutation 
in a given series. It may, indeed, be asked why Filograna, 
with such a tendency to variation in the organs mentioned, 
as well as in reproduction, has not developed along the lines 
indicated and produced descendants in which each variation 
fitted it to survive more readily than its fellows—whether 
as regards its somatic cells or its germ-cells, both of which 
are affected? The continuity of the germ-plasm does not 
appear to restrict the variations indicated, even in repro- 
duction, since there may be free ova, internal embryos, or 
buds. A study of karyokinesis in the sexual cells might, 
perhaps, aid in solving the problem, though this is 
conjectural. 
Again, the variability in the number of the anterior 
(thoracic) bristles is a feature seen in not a few Polychzts 
-—for instance, in the Sabellids. In Potamillareniformis, O. 
F. M., the anterior bristles may range from five to twenty- 
six pairs, a much wider variation than in Filograna. Certain 
races of Filograna have seven pairs, others from seven to nine, 
whilst the Neopolitan types may reach ten. Such is not 
necessarily the effect of age, but rather of environmental 
conditions—for example, the general temperature of the 
surrounding water, the rich supply of nourishment, and the 
abundance of light. 
In considering the differences in structure presented by 
Filograna the variations in the tips of the branchie occur 
under such diverse conditions, as well as on precisely the 
same site and under the same conditions, that one is at a 
loss to say wherein the process of selection and the “ struggle 
for existence” lie. Their tubes give the necessary pro- 
tection, so that the species can vary in any manner in which 
its inherent capacities permit, and a single generation may 
afford examples of change in the organs referred to. The 
enlarged tips of the branchiz are of a glandular character, 
and thus differ from the opercula—though borne by the 
same parts. Moreover, it caunot be said that disuse has 
caused the disappearance of the opercula, since the forms 
devoid of them have as much need of them as before. The 
bright pigment of the branchiz shows that light, as well as 
