Sponges are best described as aggregations
Sponges have a cellular level of organizatioi
cells are largely independent of each other at
tissues and organs (see Table 7.1, p. 148). Sp
structurally simplest multicellular animals (see
Nearly all sponges are marine. All are
manently attached to the bottom or some c
show an amazing variety of shapes, sizes, a
a relatively simple body plan. Numerous ti
on the surface allow water to enter and c
series of canals where plankton and
organic particles are filtered out and
eaten (Fig. 7.1a). This network of canals
and a relatively flexible skeletal frame-
work give most sponges a characteristic
spongy texture. Because of this unique
body plan, sponges are classified as the
phylum Porifera, or “pore bearers."
Sponges may be similar to the first
multicellular animals, which were prob-
ably simple colonies in which some cells
became specialized for such functions as
feeding and protection. Sponge cells are
very plastic and easily change from one
type to another. If experimentally sepa-
rated, the cells can even regroup and form
a new sponge (Fig. 7.2).
The architecture of sponges is best
understood by examining the simplest
kind of sponge (Fig. 7.1a). The outer
surface is covered with flat cells called
pinacocytes and occasional tube—like
pore cells, or porocytes, through which