An Introduction to the Invertebrates, Second Edition - tiera.ru

150 MOLLUSCA: BIVALVIA AND CEPHALOPODAmovement. The suboesophageal ganglion is the first of a chain of segmentalganglia along the double ventral nerve cord (see diagram). Behaviour is largelybased on segmental reflexes.Giant fibres with rapid conduction are importantin burrowing worms and in tube-living polychaetes. Leeches have morespecialised nervous systems, with the segmental ganglia condensed togetherandlarge ganglia under the suckers.MolluscsNo generalisation can be made. Gastropods typically have an anterior‘cerebral’ ganglion with very few controlling functions and about three othermain ganglia, serving the viscera, the mantle and the foot. As well as nerve fibresconnecting these ganglia there are longitudinal nerve cords, often twisted intoa figure of eight by torsion. Sessile bivalves and active cephalopods representextremes of brain development (see Chapter11).ArthropodsArthropod nervous systems are built on the same plan as those of annelids(see diagram) but arthropod brains are usually highly developed and formthe basis for most elaborate behaviour (see Chapters12 15).ChordatesUnlike other phyla, chordates have hollow dorsal nerve cords, though in invertebratechordates these may occur onlyinthe larval stages (see Chapter18).How nerves workThe basic facts are as follows:1. Electric signalling along nerves is achieved by charged ions movingin and outofthe axon (contrastthe flow of electrons along a telegraphwire).2. Passive ion movement is determined by chemical and electricalgradients.3. Axonmembranes are selectivelypermeableto sodiumandpotassiumions. There are separate sodium and potassium channels, eachregulating the passive flow ofthe relevantion.4. Axon membranes, like the membranes of nearly all cells, have inaddition a ‘sodium pump’: an active (energy-requiring) process thattransports sodium ions outward, resulting in more sodium outsidecells and more potassium inside them.Resting nerveThe cell membrane is impermeable to the large internal protein molecules,which are predominantly negatively charged. Ions move freely in and out untilequilibrium is reached. Due primarily to positively charged potassium ionsmoving out, the result is a potential difference of about 75 millivolts across