Askjagden's Guide to Biology: Sponges
Please excuse the PG-13 rating that has been set for this guide. For some reason, it cannot be changed to G. Enjoy!
The phylum Porifera stands for "pore-bearers". The members of phylum Porifera are called sponges. Sponges are the simplest animals, because the only reasons they are classified as animals are that they are multicellular, have no cell walls, are heterotrophic, and have a few specialized cells. They are porous, and these pores are very important to sponges, as you will see.
Sponges are very simple. Therefore they have no symmetry, cephalization, etc. Water enters through the pores. The movement is facilitated by choanocytes, which are cells that use flagella to move water into the sponge. Water leaves through a pore called the osculum. Sponges have a skeletol structure called spicules. A spicule is a glass-like structure made out of calcium or silica. Spicules are composed of archaeocytes. Some sponges have spongin, which are softer skeletons made of protein.
Sponges defend themselves by producing toxins that would make them lethal to predators. Respiration, excretion, and circulation can be executed by diffusion of water. Sponges reproduce by internal fertilization (internal means "inside", and fertilization is the joining of a sperm and an egg cell to form a diploid zygote, so internal fertilization is fertilization inside the body). The zygote will form into a larva. The larva will later grow into a mature sponge. Sponges can also reproduce asexually. This is possible by releasing gemmules, which are archaeocytes surrounded by spicule. Gemmules will eventually grow into mature sponges.
Sponges are sessile, meaning they are only attached to one specific spot; nowhere else. Therefore many types of symbiosis happen with sponges, because they will attach to other organisms. Sometimes, the sponges will provide shelter for other organisms. Sponges are not parasites, but can sometimes be hosts. The most common forms of symbiosis associated with sponges are mutualism and commensalism.
http://upload.wikimedia.org/wikipedia/en/5/5c/Spongebob-squarepants.png.
Have fun with biology!
