236 COMPARATIVE ZOOLOGY. 
tiation is carried to a higher degree, as we find ectoderm 
and endoderm, fibrous tissue, ciliated tracts, and a canal- 
system for circulation. 
When freshly taken from its element, a Sponge is hard 
and glistening on the outside, and strongly resembles a 
piece of liver. While living, constant currents of water 
issue from the large orifices, fed by smaller streams enter- 
ing by the minute pores, the currents being caused by cilia 
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Fia. 185.—Horny Skeleton of a Sponge. 
lining the passages. So that “the Sponge represents a 
kind of subaqueous city, where the people are arranged 
about the streets and roads, in such a manner that each 
can easily appropriate his food from the water as it passes 
along.” The apertures, or “gates,” can be closed at the 
will of the animal. 
A few species are wholly gelatinous, having no skele- 
ton; some are calcareous, aping the Corals; many are 
entirely siliceous, resembling spun glass, as the beautiful 
ee 
