|Athro, Limited Biology Evolution Scale|
|Looking closer and closer at sponges.|
Sponges are simple on the surface. They don't look like animals at all, just blobs on the sea floor with holes in them. Sure they can have lots of shapes (baskets, fingers, tubes, vases) and come in lots of pretty colors, but they just sit there on the sea floor (though there are a few fresh water sponges...) Still, if we watch a sponge closely, or pour a little milk on it (underwater of course), we see that water is continually drawn into the sponge and flows out large holes in its surface.
Looking closer at their anatomy, we see a set of chambers and tubes. Sponges continually pump water through their bodies. Water flows in through lots of little holes on the surface of the sponge (giving sponges their name:
porifera - hole bearers). It is pumped through a network of tubes branching through the body of the sponge, and flows into large chambers, and out through large holes to the outside. (Lots of other animals, like crabs and brittle stars, like to live in these chambers inside the sponge). Sponges have a skeleton. Some have a skeleton made up of durable organic fibers - spongin - the stuff of natural bath (and pottery) sponges. Other sponges have little hard needles -
spicules to stiffen their skeleton. Washing with one of these sponges would be like using sandpaper in the bath.
Looking still closer, a sponge is made up of three basic sorts of cells. There are sheets of cells that cover the surfaces of the sponge. There are cells that line that pump water and cover the walls of pumping chambers in the tubes that run through the sponge's body. There are also cells that move around inside the body of the sponge. All animals are built with this basic design of cells that form sheets (like our skin cells) and other cells (like our white blood cells) that move around the body.
Looking closer still - sponge cells are really bizarre. Sponges have many mobile cell types. Each of these cells is a specialist. Some cells wander around the body of the sponge secreting spongin to make the skeleton. Other cells secrete spicules, others pick up food from where it is captured and transport it to the rest of the body, others collect wastes from other cells. The cells that pump the water through the sponge also capture food from the water. One particular kind of mobile cell can come by, pick up some of this food, move out to the sheets of cells that cover the surface of the sponge, and feed them. This is a totally different design than we use (putting cells into tissues and organs - mouths and stomachs and blood vessels and so forth). Sponges often have single celled organisms (algae of various sorts) living within their bodies, not in the tubes, but amongst their own cells. These
symbiotic algae can be very abundant and contribute to the color of the sponge.
Looking still closer - sponge cells are built of chemicals. On a very cursory level, sponges, and other organisms, use chemicals that don't dissolve in water (fats and stuff like cholesterol) to form a cell membrane. The cell membrane keeps the world out and the sponge in. One way they keep their own chemicals in the cell is to stick charges onto them, so they will dissolve in the water inside the cell, but won't pass through the cell membrane.
The more closely we look at even the simplest organisms, the more complexity we see.