The Exterior Layer

Epidermis and dermis
With humans we may expect
And what about sponges
That nature did not neglect

For it gave it special skin
And pores that open wide
Learn about exterior cells
Before we look inside

 

The exterior layers of sponges is implemented for the objective of absorbing water with useful substances and repulsing waste water that has already been filtered. The purposes of the each of the sponge's exterior cells is to provide a service, however minute it may be, to the general welfare of the porifera. When these services are combined, the ultimate task is achieved- sustaining life. Here is a listing and description of the main, currently known types of exterior sponge cells. (Look at the diagram for comprehension.)

1) Pinacocytes- These are referred to, in human terms, as skin cells. They provide a thin and tight layer of coverage for the spongy fibers beneath, which is actually very much like skin. However, it is rather leathery. Spicules, originating within the interior layers, protrude through the pinacocytes, outside of the exterior layer. In this way, they provide a far more efficient means of protecting the sponges and preserving their structure. However, both pinacocytes and spicules may be used in chemical "war fare."

2) Porocytes- Throughout the pinacocytes, at various intervals, there are porocytes which are each associated with a pore on the surface of the sponge. Porocytes open the pores and are, in descriptive terms, the "doors" to the passageways, or canals, of the sponge. Porocytes may complement incurrent (Latin= running in) and excurrent ducts.

3) Myocytes- Although sponges have no muscles or organized tissues, the myocytes produce movement through constriction and contraction. Myocyctes regulate the opening and closing of the porocytes. When certain pressures are formed over ducts, the myocytes open up, and therefore, the porocytes open up. Myocytes and porocytes function with each pore. A pore is also called an ostium. This is Latin for mouth.

4) Osculum- The osculum is actually a lack of cells. In more simple terminology, it is the hole, generally at the top of a porifera, that releases waste materials. It is the most significant excurrent duct of a sponge. Ostia are smaller holes on the sides.

With the collaboration and cohesion of the cells of the exterior layers, a sponge attains and maintains homeostasis- complete balance.