CANAL SYSTEMS AND SKELETAL STRUCTURES
If it had not been for the
development of elaborate canal system, sponges would have remainedin the simple
asconoid condition of leucosolenia and would never have been able to become
massive in size. The canal system furnishes an avenue for food through the body
and for transportation of exretorymatter out of the body. Three types are
usually recognized (fig. 7.2): the siplest or ascon type, as in leucosolenia;
the sycon type, an in scypa; and teh leucon type, (rhagon) type, in which there
is a number of small chambers lined with choanocytes.
The skeletons of sponges consist of
calcium carbonate or silica (a mineral substance akin to glass) in the formof
spicules, or of spongin in the form of protein fibers more or less closely
united (fig. 7-3). Spongin is secreted by flask-shaped cells (spongoblasts). Spicules
are deposited in cells, and more than one cell may take part in the formation
of a single spicule.
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