SPONGES 95 
SKELETON, 
The skeleton of the group of sponges which we have under consideration, 
{s composed of horny matter, which increases in thickness with the age of the 
sponge, hence older sponges are of a firmer structure than when young. In 
the Tube Sponge, in which the fiber of the skeleton is quite large, we find one 
of the best species for study, 
The general color of the skeleton is amber, light when young, but darken- 
ing with age. ‘The fibers are cylindrical, well rounded and seldom flattened 
even at the points of the jointure. They are hollow and at the extreme tips of 
the new growths are composed of a single, soft membrane which is yellow in 
color and opaque. ‘This soon becomes covered with a layer of pale, amber- 
colored, horn-like matter which is nearly transparent, and successive layers of 
this horny matter are deposited ; thus the older growths not only become larg- 
er in diameter, but darker in color on account of the continuous thickening of 
the deposits, (See Figs, 14 and 16 where I have given an enlarged cut of the 
fiber, a, being the new in A, C and B in Fig, 14, and y the old ; then compare 
with the old growth in Fig. 14 C ,in this the original hollow membrane may 
be seen through the transparent walls. 
New twigs of the fiber arise from division at the extreme tips of the new 
erowths. (See Fig. 16 C, a, and Fig. 14 a.) The twigs thus formed, al- 
though of an equal length at first, do not always remain so, for the onward 
growth of one may assume a different direction from that of the other, so that 
it may soon come in contact with the tip of another tube, forming a comparativel y 
