60 SPONGES 
perfectly natural. One m which the spicules are composed wholly of silica, 
and one in which the spicules are calcareous or composed of lime. As far as 
recent sponges are concerned, the hne between these groups is sharply drawn ; 
that is, we do not find any sponges in which the spicules are composed partly 
of silica and partly of lime; they are composed wholly of one or the other. 
Hence we have first : — 
SILICIOUS SPONGES. 
Sponges m which the spicules are composed wholly of silica. This group. 
by the environment, is divided into two groups; Salt Water Silicous Sponges. 
and those which inhabit fresh water. 
SALT WA’PER SILICOUS SPONGES. 
These sponges vary from; First, in which the spicules are scattered spar- 
ingly over the horny net-work of the fiber, as seen in the Gray Cup Sponge 
( Fig. 23, page 38 } a species in which the homy fiber is drawn outm long 
thread-like filaments, which run parallel to one another, and which bend out- 
ward at their terminations. From these filaments projeet the spicules, much 
as do the thorns on a rose bush. See Fig. 33, B, where I have givena cut of 
these fibers much enlarged. 
Second, species in which the sponge is made up nearly wholly of pure 
silica. Examples of this kind of sponge may be seen in the Glass Sponge, and 
in the well known Venus Flower Basket ( Euplectella ) which is composed of 
long parallel filaments of silica connected by other cross filaments, As all of 
