Sponges have a cellular grade of organization. They do not possess any structures that
can be considered organs. For instance, sponges do not have stomachs or kidneys.
Instead, sponge cells of various types are responsible for bodily functions,
the day-to-day activities that sustain life. Many of the most common
types of cells are illustrated below in a cartoon view of the wall of a poriferan.
Pinacocytes
These cells are the "skin cells" of sponges. They line the exterior
of the sponge body wall. They are thin, leathery and tightly packed together.
Choanocytes
These distinctive cells line the interior body walls of sponges.
These cells have a central flagellum that is surrounded by a collar of microvilli.
It is their striking resemblance to the single-celled protists called
choanoflagellates
that make many scientists believe that choanoflagellates are the sister group to
the animals.
Choanocytes are versatile cells. Their flagella beat
to create the active pumping of water through the sponge, while the collars of
the choanocytes are the primary areas that nutrients are absorbed into the
sponge. Furthermore, in some sponges the choanoflagellates develop into
gametes.
Mesenchyme
Between the two layers is a thin space called mesenchyme or mesohyl. The
mesenchyme consists of a proteinaceous matrix, some cells, and spicules.
Archaeocytes
Archaeocytes are very important to the functioning of a sponge.
These cells are totipotent, which means that they can change into
all of the other types of sponge cells. Archaeocytes ingest and
digest food caught by the choanocyte collars and transport
nutrients to the other cells of the sponge. In some sponges,
archaeocytes develop into gametes.
Sclerocytes
The secretion of spicules is carried out by sclerocytes.
Other cells, called spongocytes, secrete the spongin skeletat fibres
when those are present.
Myocytes and Porocytes
Poriferans do not have any muscle cells, so their movement is
rather limited. However, some poriferan cells can contract in a
similar fashion as muscle cells. Myocytes and porocytes which surround
canal openings and pores can contract to regulate flow through the sponge.
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