Invertebrate Nervous Systems


The nervous systems of invertebrates are smaller and contain fewer neurons than those of vertebrates, yet they generate the great diversity and complexity of behaviour that has contributed to the incredible success of these animals.

Keywords: brain; invertebrate; neuron; nervous system; behaviour

Figure 1.

Different types of invertebrate nervous systems. (a) A simple nerve net of loosely associated neurons in a sponge connected at points of contact by synapses (high magnification view). (b) The ladder‐like layout of a flatworm, with two longitudinal connectives running the length of the body, joined at intervals by commissures. (c) The segmentally organized nervous system of an annelid, with the longitudinal connectives and ganglia fused along the midline. There is an elongated ganglion for each segment. (d) Three ganglia located in the segmented thorax of an insect. The ‘metathoracic ganglion’ is really a fused structure containing neurons of the metathoracic ganglion and the first three abdominal ganglia.

Figure 2.

Cross‐section through an insect ganglion to show longitudinal bundles (tracts) of axons, neuron somata in a layer around the margin, commissures containing branches of neurons that cross the midline, and the remaining areas of neuropil where almost all of the fine branches and synapses are found. In a large insect like a locust, a ganglion measures about 1 mm across.


Further Reading

Barnes RSK, Calow P, Olive PJW, Golding DW and Spicer JI (2001) The Invertebrates: A Synthesis, 3rd edn. Oxford: Blackwell Science.

Bullock TH and Horridge GA (1965) Structure and Function in the Nervous Systems of Invertebrates. San Francisco: Freeman.

Burrows M (1996) The Neurobiology of an Insect Brain. Oxford: Oxford University Press.

Gilbert SF and Raunio AM (1997) Embryology. Constructing the Organism. Sunderland, MA: Sinauer.

Lacalli TC (2001) New perspectives on the evolution of protochordate sensory and locomotory systems, and the origin of brains and heads. Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences 356 (1414): 1565–1572.

Mackie GO (1999) Nerve nets. In: Adelman G and Smith BH (eds) Encyclopedia of Neuroscience, pp. 1299–1302. Amsterdam: Elsevier.

Maddison DR and Schulz K‐S (eds) (2002) The Tree of Life. http://tolweb. org/tree/phylogeny html [An excellent gateway to information on most animals.]

Ruppert EE and Barnes RD (1994) Invertebrate Zoology, 6th edn. Fort Worth: Saunders College.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Matheson, Thomas(Oct 2002) Invertebrate Nervous Systems. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0003637]