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http://www.ucmp.berkeley.edu/porifera/porifera.html
Article from UC - Berkley on the Poriferans which are commonly referred to as sponges. An early branching event in the history of animals separated the sponges from other metazoans. As one would expect based on their phylogenetic position, fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian.

http://www.oceanicresearch.org/sponges.html
Gives an essay on this animal including a cross-section picture, types of sponges, and reproductive details.