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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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Bryozoa (Ectoprocta)

Bryozoa (Ectoprocta)

(p.325) 26 Bryozoa (Ectoprocta)
Structure and Evolution of Invertebrate Nervous Systems

Alexander Gruhl

Thomas Schwaha

Oxford University Press

Bryozoa (or Ectoprocta) are a taxon of sessile aquatic suspension feeders with about 6000 described species, living in marine and about 100 in freshwater habitats. Bryozoans form clonal colonies composed of numerous millimetre-sized individuals, called zooids, which maintain tissue interconnections. Traditionally, two main body regions are distinguished in an individual zooid, the protective cystid, which harbours a spacious fluid-filled coelom and is attached to the substrate or to neighbouring zooids, and the distal polypide, which consists of the tentacular crown, the tentacle sheath, and the digestive tract. The brain (also referred to as cerebral ganglion or ganglion) is located in the base of the lophophore. Underlying and innervating the ring of tentacles are large nerve cords that embrace the pharynx and form the so-called circumpharyngeal nerve ring. Additional neurite bundles emerging from the lateral and basal parts of the brain innervate further epidermal areas, the intestine, the musclature, and coelomic epithelia. Adult bryozoans lack complex sensory organs, but have many mechano- and possibly chemoreceptor cells on their tentacles and other uncuticularized parts of their body wall. Photoreceptors have not been demonstrated, although some colonies show positive or negative phototropism. Pigmented ocelli occur in some but not all larvae. Larvae also bear more or less complex apical organs, composed of several ciliated sensory cells, and frontal pyriform organs comprising sensory and gland cells.

Keywords:   Bryozoa, brain, circumpharyngeal nerve ring, apical organ, photoreceptors

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