421 
Bibliographical Notice . 
Darivinella, F. Muller, which possess a skeleton mainly of horny 
spicules detached from each other and irregularly scattered in the 
mesoderm of the sponge. Only two species are as yet known : in 
the first described, D . aurea , the spicules have from three to eight 
rays ; some of them resemble the four-rayed or Calthrops spicules 
of siliceous Tetractinellid sponges, whilst others approach in form 
the six-rayed spicules of Hexactinellids. In the other species, D. 
australiensis , Carter, the large majority of the spicules have only 
three rays in one plane, and thus singularly resemble in form the 
three-rayed spicules so common in Calcisponges. The author con- 
cludes that these varied forms of horny spicules in Darwinella are 
directly derived from the siliceous spicules of the Hexactinellida in 
which the silica has been replaced by spongin ; but there seems 
very little warrant for supposing that spicules so far removed from 
the Hexactinellid type as the Calthrops and three-rayed forms can 
ever have been derived from normal six-rayed Hexactinellid 
spicules ; if they have been derived from siliceous sponges at all, 
they are more nearly related in form to Tetractinellid spicules. 
Yet further, Hr. Lendenfeld states that the substitution of spongin 
for silica in these horny spicules has been brought about to meet 
the “ exigencies of chauged circumstances resulting from a migra- 
tion from the siliciferous depths of the ocean to shallower water, 
where the amount of silica contained in solution in the water is not 
so great ” ! It may be asked if there is any reason for believing 
that the water of the ocean at great depths contains more silica than 
in shallower areas ? Judging from the abundance of recent siliceous 
sponges in shallow and moderate depths, and from their enormous 
development under similar conditions in past ages, there is no ground 
whatever for supposing that the spicules of siliceous sponges would 
be at all likely to undergo substitution of spongin for silica through 
a comparative scarcity of this mineral in shallow water. 
The author justifies the inclusion in Horny Sponges of such 
genera as Halisarca and Bajulus , in which there is no horny skele- 
ton whatever, on the ground that they are rudimentary horny 
sponges ; on the other hand, Schulze considers these forms as rudi- 
mentary Hexactinellids ! 
A total number of 248 distinct species and varieties are described 
in this work, of which no fewer than 258, or 74 per cent , are found 
in the Australian seas, whilst 179 species are limited to this region. 
Horny sponges are distinctively inhabitants of shallow water, the 
greater number occurring at depths between 20 and 50 metres, and 
the greatest depth at which they have been met with is 750 metres. 
They also flourish most in warm seas. 
In the synthetical part of the volume the general results deduced 
from the empirical descriptions are discussed in a series of chapters 
in which the structure, classification, and systematic positions of 
sponges generally are treated. We can here only touch upon a few 
salient points, and one of these is the statement that the canal- 
system is the most important organ in sponges, and that it should 
principally be taken into account in classifying them. But is it not 
the fact that an essentially similar canal-system is present in many 
