Why are sponges considered colonial organisms?

Sponges are simple animals in the Phylum Porifera. They
have two layers of cells with a jelly-like material sandwiched between. They do not have
highly specialized systems in the body. Rather, they rely on water flow throughout the
sponge transporting food, oxygen and wastes. When they reproduce, they can carry out
both sexual and asexual reproduction. During asexual reproduction, if a piece of a
sponge breaks off, it can reattach someplace else and form a new colony. When well-fed,
a sponge will asexually reproduce by forming identical buds. These may remain attached
to the colony, or break off and form new colonies. Sometimes, sponges send out gemmules,
or survival pods, when conditions are not quite perfect for their survival. Sometimes,
these may merge with cells of other sponges from the same species to form a new
colony.