Sponges - or from fragments or sexually Sponges produce...

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Sponges: The Phylum Porifera Modern sponges greatly resemble some fossil Cambrian sponges. Sponges may have evolved from a colonial protozoan, as shown in Figure 8. There are no true tissues in sponges: merely specialized cell layers. Epidermal cells in sponges line the outer surface. Collar cells line the inner cavity. Beating collar cells produce water currents that flow through pores in sponge wall into a central cavity and out through an osculum, the upper opening. A 10 cm tall sponge will filter as much as 100 liters of water a day. Amoeboid cells occupy the "inner" layer, along with hardened structures known as spicules . Sponges feed by drawing water into the body through a network of pores (hence the name porifera , pore-bearer) and passing it out through the large opening (osculum) at one end of the body.
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Sponges can reproduce asexually (by budding
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Unformatted text preview: or from fragments) or sexually . Sponges produce eggs and sperm that are released into a central cavity of the sponge, in which the zygote develops into a ciliated larva. The larval stage is able to move about while the adult is stationary. The fossil record of sponges has been at times quite good. The oldest sponges date from the precambrian. One early example of fossil sponges are the archaeocyathids, one of the first reef-building animals. Archaeocyathids evolved and went extinct before then end of the Cambrian Period. Cladistic analysis by J. Reitner in 1990 suggests archaeocyathids are properly placed in the Phylum Porifera instead of having their own phylum. Living sponges fall into three groups: the calcareous (an example of which is shown in Figure 9), glass, and demosponges, based on the chemical composition of spicules....
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