396 Dr. W. Marshall on new Stliceous Sponges. 
cuce in them a sort of summer sleep, necessary for the main- 
tenance of the species, which they pass either by the whole 
organism being in a state of latent vitality, or by breaking up 
into descendants (in the form of fragments of themselves). 
What the desiccating heat of the sun is capable of doing in 
warm regions is effected in colder ones by the severity of 
winter ; in both cases the animals are deprived of a part, and, 
indeed, a principal part, of their conditions of existence, food 
and moisture or heat, and in the two cases a similar ‘result 
originates from partly antagonistic causes—side by side with 
the summer sleep of the tropical or subtropical organisms we 
get, as is so frequent, a winter sleep of those inhabiting colder 
zones. 
The freshwater sponges, as is well known, do not fall in 
their entirety during the summer or winter sleep into a latent 
state of existence; this mode of persistence 1s perhaps con- 
ceivable only in forms living at very great depths, which are 
but little exposed to desiccation or to the action of cold. As 
whole organisms they for the most part disappear, and as 
they partially break up into germinal fragments, their modi- 
fied seasonal sleep leads to a form of reproduction. 
These germinal fragments, if they remained naked as when 
they are formed, would soon succumb to the power of the 
heat or cold. What had been acquired for the maintenance 
and increase of the species would be but badly preserved if 
from the first there had not also been acquired suitable 
defensive arrangements against these climatic influences, in 
the shape of shells which, i in this respect quite analogous to 
the shells of eggs, could protect the germ as much as possible 
from freezing or desiccation, in short from destruction. Of 
such protected winter-eggs and winter-germs many occur, as 
is well known; and it would be interesting to know whether, 
in double-brooded insects for example, one brood of which 
passes the winter in the egg (there seem not to be many of 
them), these winter-eges differ from the summer- ege’s in the 
thickness of the shell, the mode of sheltering on the part of 
the mother, &e. 5 in other words, whether there is a seasonal 
dimorphism of the eggs. Unfortunately I have been unable 
to find any statements upon these points in literature; but 
Werneburg * says of the shells of the eggs of Lepidoptera i in 
general that they differ in relative thickness and ‘‘are strongest 
in those which he uncovered, and among these are particularly 
strong in those which remain undeveloped through the winter. 
Thus, for example, the eggs of Bombyx neusiria, which re- 
* Der Schmetterling und sein Leben:’ Berlin, 1875, p. 46. 
