Lecture 10.docx

Lecture 10: Multicellular Organisms – The Way of Animals 2 Flatworms and the Advantages of Bilateral Symmetry Bilateral symmetry allows for more directed movement and tracking Where did bilateral symmetry come from? Not from adult cnidarians, but evo-devo The planula larvae of cnidarians have the ru...

Lecture 10: Multicellular Organisms – The Way of Animals 2 Flatworms and the Advantages of Bilateral Symmetry Bilateral symmetry allows for more directed movement and tracking Where did bilateral symmetry come from? Not from adult cnidarians, but evo-devo The planula larvae of cnidarians have the rudiments of bilateral symmetry, so a likely ancestor of bilateral animals is a cnidarian larva that retained its immature form but gained sexual maturity. Protostome: Blastopore becomes mouth, 2nd pore become anus Nerve cord runs ventrally Deuterostome: Blastopore becomes anus, 2nd pore becomes mouth Nerve cord runs dorsally Enter Flatworms Bilateral symmetry allows concentration of neural tissue and sensory systems at one end (head) Easier to achieve directed, targeted movements, which as seen in flatworms can facilitate hunting and reproduction But there are limitations: Tissues and body cavities Bauplane Combining type of symmetry, tissue types, body cavities and other traits leads to the unique Bauplan of each animal phylum The Exception that Proves the Rule “Proves” comes from the Latin “provare”, meaning ‘to test or try’, so the saying’s original meaning was “the exception that tests the rule” What is the exception? Echinodermata If being bilateral is so advantages, why re-evolve radial symmetry? Deal with multiple threats and opportunities around you. But by doing so you lose directionality. Solution: The arm closest to the threat/opportunity takes the lead role. But to do so each needs to be equipped with a sensory system, an effector system, and a non-centralized nervous system, so no brain E.g., Eyespots Neurons – A Revolution Speed and specificity Synapses Pre/post synapse Interconnections – Interneurons start regulating the flow of information Sensory, Relay, Motor Think of nerve ganglia as a cluster of interneurons Think of the brain in bilateral animals as an expanded ganglion (i.e., more interconnections) The brain maximizes the revolution brought by neurons by increasing the number of options in processing sensory inputs, using stored information and varying the way effects can be manipulated All animals, especially those with brains, have the ability to engage in all forms of behavior recognized in the traditional schema