*** Appendix - The nervous system and sensory capacities of Cnidaria (jellyfish, sea anemones, corals and their relatives)

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The box jellyfish Chironex fleckeri. Courtesy www.barrierreefaustralia.com.

Relationship to other animals

The subkingdom Eumetazoa (so-called "true" animals, which excludes sponges) is made up of 33 phyla; most have tissues organized into organs and organ systems (World Biodiversity Database, 2000). The simplest of these phyla are the cnidaria (commonly known as coelenterates, including animals such as jellyfish, sea anemones, corals and freshwater hydra). Cnidaria have a body wall that is made up of only two layers of cells, like that of sponges; other "true" animals (eumetazoa) have three. Unlike sponges, cnidaria possess a basic kind of body symmetry: they are radially symmetrical.

Nervous system

Nervous systems and brains are unique to animals; they are not found in any other kingdom of organisms (World Biodiversity Database, 2000). Cnidaria have a rudimentary nervous system, with neurons positioned regularly over the surface of the animal. Each neuron is in contact with its neighbours. The propagation of a nerve impulse is not transmitted along a linear chain of neurons, but radiates from its point of origin (Abramson, 1994, p. 176).

Although cnidaria do not possess a central nervous system, let alone a brain, their nerve net permits rapid communication between cells (in some cases taking only milliseconds), over relatively long distances. In "simpler" animals, which lack neurons, communication can only occur between neighbouring cells. Prescott considers the nervous system they possess to be a major advance in the evolution of animals:

Whilst the most primitive metazoans [i.e. animals - V.T.], the sponges, lack neurons and respond only to direct stimulation (usually with a very slow, spreading contraction), cnidarians have quite complex nervous systems, composed, principally, of distributed nerve nets, and show both internally generated rhythmic behavior, and co-ordinated patterns of motor response to complex sensory stimuli.... Most of the neurophysiological features of more 'advanced' metazoan nervous systems are actually present at the cnidarian grade including multifunctional neurons, action potentials, synapses, and chemical neurotransmission... In ... the hydrozoan jellyfish, parts of the nerve net are fused to form longitudinal or circular tracts that allow very fast signal conduction and can support fast attack, escape, or defense reactions (2001, pp. 5 - 6).

Rodney Cotterill. Picture courtesy of Danish Technical University.

According to Cotterill (2001, p. 5), most jellyfish exhibit a sluggish response to stimuli, but two types of jellyfish exhibit dual reponse patterns: slow feeding as well as the ability to rapidly escape from predators, which Cotterill considers to be a genuine autonomous reflex.

Some cnidaria - e.g. sea anemones and the jellyfish Aurelia aurita - possess a nerve net that is functionally divided into two relatively independent systems: one for feeding and the other for movement. In the jellyfish Aurelia aurita, the two systems make contact in neuron clusters called marginal ganglia. Each ganglion is a part of two networks: the network that controls feeding behaviour and a second network that regulates swimming. Because it is part of the swimming network, each ganglion has a regular beat, but it can also generate its own rhythm if isolated from the network. Other cnidaria, such as the jellyfish Aglantha digitale, have a different arrangement: "a single nerve net which can carry two different types of action potentials enabling either rapid escape swimming, or, slow rhythmic swimming for feeding" (Prescott, 2001, p. 6). Aglantha digitale has a giant axon with very fast conductance, so that a nervous impulse can traverse the circumference of the bell in a few milliseconds.

Sensory capacities

Cnidaria possess a variety of sensory capacities:

Many of the free-living cnidarians also possess light-sensitive and gravity-sensitive organs that allow behaviors such as orientation, sun compass navigation, and daily migration (Prescott, 2001, p. 6).

However, we have already discussed the sensory capacities of bacteria, protoctista and plants, and argued (Conclusion S.3) that sensory capacities per se do not require a mentalistic explanation: once can describe them using a mind-neutral, third-person intentional stance.

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