THE COMMON FEESH-WATER SPONOE. 
3 
is only by the combined labours of many observers that their 
various phases of life and structure are harmonised. At present 
we are mainly dependent upon the excellent observations of 
Carter and Lieberkuhn, but much further inquiry will be needed 
before the history can be completed. 
The skeleton consists mainly of numerous minute siliceous 
spicula, combined to form small irregular rods (fig. 3), which 
join each other at obtuse angles, to produce a very irregular 
network (fig. 2). Whether the spicula are bound together into 
rods, and the rods into a network, by any other material than 
sarcode is doubtful ; no other cement has hitherto been satisfac- 
torily shown to exist. Most of the spicula are long, slender, 
and smooth, pointed at each end (fig. 4), but others exist less 
abundantly in which the surface is muricated (fig. 18u). 
The gelatinous investment, or sarcode, consists of an aggre- 
gation of numerous atoms which may be termed sarcoids^ each 
one closely resembling a true Amoeba in many of its aspects and 
actions. Various observers have noted that when the sarcode is 
broken up into fragments, in water, each portion is a more or 
less tremulous mass of sarcoids, and displays cilia projecting 
from its outer margin (fig. 5). Each sarcoid (fig. 6) exhibits 
movements resembling those of Amoeba, pushing out portions of 
its substance and retracting others. According to Lieberkuhn, 
these movements are confined to the non-ciliated portions of the 
sarcode. Lieberkuhn terms these separate atoms cells^ explain- 
ing, however, that he has never seen them invested with a true 
cell membrane. This use of the term is objectionable, though 
probably each sarcoid may be regarded as identical with the 
protoplasmic mass upon and by which the true cell membrane is 
formed, in both the animal and vegetable kingdoms. That such 
is the case is the more probable, since each sarcoid contains at 
certain seasons a nucleus and nucleolus ; but for the present the 
use of the objectionable term is unavoidable. Each so-called cell 
is about diameter, the nucleus being yxVo? 
nucleolus q-o^q-q. Thus we may legitimately regard the sarcode as 
being virtually a mass of Amoeban sarcoids, combined to form a 
compound organism. We shall recur to the structure of the 
sarcode in describing some of Mr. Carter’s important observations 
on the development of Spongilla. 
We have already referred to the existence of numerous large 
seed-like bodies (fig. 2h) in the interior of the organism — 
objects commonly sold by dealers in microscopic objects under 
the name of spores, or gemmules of Spongilla. These are the 
most curious and characteristic features of Spongilla, and require 
a more elaborate description. 
They are spherical bodies (fig. 7 ), with a small spot or aperture 
(fig. la) called the hilum on one side. I have observed that. 
