|Lucy meets a distant relative|
|A typical sponge? Note the chimneys (exhalent canals) and small pores (inhalent canals).|
Fundamentally, though, a sponge is a living sieve. It's a body with pores all over it, leading to canals that join together, eventually leading to one or more large holes at the top. The canals are lined with cells that have little whip-like flagellae, which all beat in the same direction to suck water in through the pores, and spit it out at the top. Sponges may not have organs, it's true. But they do have a surprising array of different cell types. Some of them create currents and catch food, others are for oxygen exchange, others for secreting the skeleton, others for reproduction and so on. Some of them, the archaeocytes, can become any other type - a very neat trick similar to the stem cells so beloved of modern medicine.
|The organic (collagenous/chitinous) skeleton of a modern "bath sponge"|
I mentioned a skeleton, and here's the one of a typical bath sponge. It's made of collagen fibres forming a network, and this supports the sponge's soft tissues. However... it's not normal. Most sponges have a skeleton made of interwoven or fused spicules of either opal-A (amorphous silica, similar to quartz, similar to glass, but not quite either) or calcium carbonate (calcite). This is a bit of the skeleton of Euplectella ("Venus' Flower Basket") - it's an amazingly exquisite construction of rods and cross-shaped spicules woven together:
|This file is licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license|