RHIZOPODA. 43 
were pretty generally regarded as being vegetables, and it is 
only since the microscope has been employed in their elucida- 
tion that their true nature has been made out. Most naturalists. 
are now agreed as to the propriety of placing the sponges in the 
animal kingdom, and they are generally referred to the R/zzo- 
poda, though they are sometimes looked upon as constituting a 
distinct and separate class of the Profozoa. The apparent com- 
plexity of structure which the sponges exhibit is due to the fact 
that what we ordinarily term a sponge is really a colony or 
aggregation of separate masses of sarcode, greatly resembling 
Amebve in structure, and having the power of secreting a 
skeleton or supporting framework common to the whole assem- 
blage. Sponges, in fact, may be defined as compound Rhizo- 
poda, forming masses which are traversed by canals opening on 
the surface, and supported by a framework of horny fibres or of 
calcareous or flinty needles. 
There are, then, two essential elements in the structure of a 
sponge—namely, the sarcode-bodies which constitute the ani- 
mal itself, and which are collectively termed the ‘‘sponge-flesh,” 
and the hard framework or “skeleton” upon which the flesh is 
supported. To understand the nature of these fully, we may 
take an ordinary horny sponge, such as we are constantly in 
the habit of using. As we see the sponge in this country, we 
are only acquainted with the skeleton, which is composed of an 
enormous number of horny fibres, all interlaced and interwoven 
with one another, but leaving numerous holes and canals 
between their bundles (fig. 11, @). In its living condition, 
however, the whole of this skeleton is covered inside and 
outside—saturated, in fact—with a kind of slimy material very 
like white-of-egg to look at. This is the so-called sponge-flesh ; 
and, upon examining this with a microscope, it is found to be 
composed of an enormous number of minute masses of sarcode, 
all more or less completely independent of each other, and each 
very closely resembling an Ameba. These separate “ sponge- 
particles,” or “sarcoids,” as they are called, consist, in fact, of 
granular sarcode, capable of pushing out little processes or 
threads of sarcode in the form of pseudopodia, and sometimes 
furnished with an internal solid mass or nucleus (fig. 3, ¢). In 
some cases each sarcoid carries a single lash-like vibrating fila- 
ment or cilium (fig. 3, d, ¢). Each sarcoid has the power, if 
detached, of independent movement, and each can obtain food 
