The leech is a type of worm called an annelid, which has a body divided into similar parts, or segments. Annelids include marine bristle worms (class Polychaeta, 10,000+ species), earthworms (class Oligochaeta, 10,000+ species), and leeches (class Hirudinea, 700 species). Most leeches are blood-sucking parasites and attack a variety of hosts, including invertebrates, fish, amphibians, reptiles, birds, and mammals.

The leech's nervous system is made up of a brain, a nerve cord, and 21 ganglia located in each segment along the nerve cord.The location of neurons within each ganglion is consistent from ganglion to ganglion, and from leech to leech. Each ganglion has approximately 175 pairs of large neurons (50–80 μm), which makes it relatively easy for scientists to identify specific neurons.

Leech neurons that respond to tactile stimuli can be classified into different types based on how they respond to stimuli and their anatomy. For example, one type of neuron is a T (touch) neuron. T neurons with similar shapes and response patterns are found in different ganglia, in different individuals, and even in different species.