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Structure and Evolution of Invertebrate Nervous Systems$
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Andreas Schmidt-Rhaesa, Steffen Harzsch, and Günter Purschke

Print publication date: 2015

Print ISBN-13: 9780199682201

Published to Oxford Scholarship Online: March 2016

DOI: 10.1093/acprof:oso/9780199682201.001.0001

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(p.48) 6 Ctenophora
Structure and Evolution of Invertebrate Nervous Systems

David K. Simmons

Mark Q. Martindale

Oxford University Press

The ctenophore nervous system is very complex with various specialized sensory cells, peripheral nerve nets, a centralized apical organ, neurons identifiable in all animal layers (epidermis, mesoglea, and gastrodermis), and with extensive regional specialization. The genomic content of nervous system genes (neural transcription factors, pre- and post- synaptic scaffold, neurotransmitter enzymes, and axon guidance genes) in ctenophores is the most reduced of any animal, more similar to that of the sponges, than to placozoans, cnidarians, and bilaterians. It is curious why ctenophores, but not sponges and placozoans, have such a highly diverse nervous system, even though sponges and placozoans have a larger number of genes traditionally thought to be involved with neural development and function. The proposed phylogenetic placement of the ctenophore divergence, at the base of the Metazoa, suggests that some form of a nervous system was already present at the ctenophore-extant animal split, and that this nervous system became exquisitely adapted for life as pelagic gelloplankton in ctenophores; while sponges and Trichoplax failed to exploit or lost this system. Conversely, ctenophores could have independently evolved their nervous system from the generic genetic components pre-existing in its extinct metazoan ancestors, and nervous systems were invented again by the cnidarian–bilaterian ancestor. Regardless, our current understanding of nervous system structure and genomic data from bilaterians, all four non-bilaterian phyla, and various protists, strongly suggests that ctenophores possess the absolute minimum genetic requirements for a functional nervous system of any animal. Consequentially, ctenophores are also most likely the oldest extant animal to possess a functional nervous system.

Keywords:   ctenophora, nerve net, apical organ, reduced neural gene content, minimum genetic requirement

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