BIOLOGY 3 Lecture Notes - Lecture 31: East Los Angeles College, Nitric Oxide, Nerve Net
Document Summary
Evolutionary relationships of the five main branches of animals based on recent phylogenetic analyses. Hypothetical scenarios for the loss of neural cell types in sponges. A nerve-net type nervous system in ancestral sponge may have provided a physical obstruction to maximal filtering efficiency. Losing this nerve net may have been selectively advantageous. The metabolic cost of neural mechanisms is high. The loss of neural cell types may have provided a selective advantage by allowing for the reallocation of resources to filtering activities. Ancient endoparasitic lifestyle of porifera and/or placozoa. It is possible that the stem ancestors of either poriferans or placozoans lived as endoparasites, and that some event led to them reverting to a free-living lifestyle. As parasitism can be associated with loss of many cell types including neurons [59], this hypothetical scenario might explain why it is not possible to represent neural cell types in poriferans and placozoans.
