Sponges
Types of Sponges
Glass sponges
Calcareous sponges
Demosponges
Homoscleromorpha
Microbiology of Sponges
3 layers: central jelly layer surrounded on either side by live cells.
No nervous system (this makes them the simplest multicellular creatures in the animal kingdom, putting them above jellyfish, who have a similar cellular structure AND a nervous system).
Jelly layer is non-living, but contains a variety of other cells.
Outer cell layer is made of pinaccocytes, forms external skin.
Inner cell layer is made of Choanocytes, which regulate water flow
Porocytes regulate water filtration in the ostia, which cross the Jelly layer.
Archaeocytes are totipotent (can transform into any other type of cell in the body), useful for regeneration, reproduction, hazard removal, transport, and a variety of other tasks.
Sclerocytes secrete spicules (little sharp bits of a sponge, make it harder to eat).
The life of a sponge
Sponges gain basically all of their nutrients and sustenance from water filtration.
Most have to remain moored to a solid bottom, but some can moor themselves on soft bottoms (sand, etc) via a thick root system (looks a lot like a tap root in plants).
Clogged porocytes+ostia are cleaned by Archaeocytes.
Sponges have a basic immune system that prevents cell movement in infected areas, and eventually kills all cells in the area by excreting toxin.
Sponges can reproduce both sexually and asexually. Most marine sponges don't willingly produce asexually, but shards can regenerate into new sponges if they are broken off of the main body.
Most marine sponges release sperm and hold onto eggs. When another sponge's sperm comes into their filtration system, it is absorbed and carried to their eggs. The new sponge grows out of the side of the parent and eventually removes itself.
River sponges reproduce asexually all the freaking time because it's an effective migration method in a river. They basically fertilized themselves, then let the "zygote" develop into a ball of archaeocytes with lots of food, then put it in a dormant state that's adaptable to super-wide temp+salinity+sediment+etc ranges, then release it. A few marine sponges do, but I don't think it's as effective when currents are circular in nature and always going towards the nearby shore.
Ecological Interaction of Sponges
Sponge filtration is actually a vital part of the coral ecosystem, as many sponges can cycle great amounts of nutrients back into the ecosystem very quickly after fish, corals, etc, die and degrade (normally disintegrated resources/floating detritivores might be carried away by the current, or any number of other things).
This is part of why Darwin's paradox is possible.
Sponges are very good at competing for space, which makes sense since they can't actually move. They shed their spicules, making it hard for predators to approach them (think surrounding your tent with caltrops/upright nails/legos every night before going to sleep in the woods). They also produce toxins that make life difficult for other nearby sessile organisms.
Resources
http://www.sciencemag.org/content/342/6154/108.full
http://www.nature.com/nature/journal/v413/n6857/full/413726a0.html
http://www.nature.com/news/2009/091116/full/news.2009.1088.html
http://news.nationalgeographic.com/news/2001/11/1107_keyholecoral.html
http://link.springer.com/article/10.1007%2Fs10750-011-0842-x
http://www.qm.qld.gov.au/Find+out+about/Animals+of+Queensland/Sea+Life/Sponges/Unique+features+of+sponges#.UwA1doXPuh4 (this has a majority of the basic information in it)
NOTES
Freshwater sponges probably don't matter on here, but holy hay are they different.
Resource URLs can turn into actual citations later.