Neuron type ( Bota & Swanson, 2007 ) : For purposes of wiring diagrams, neuron types are defined and classified primarily by their connections and, secondarily by their location, shape, and size. Neurons (Waldeyer, 1991) can be placed in a seven-level taxonomic hierarchy, with neuron types occupying a level equivalent to species in animal taxonomy. Formally, a neuron type is defined as a distinct cluster in polythetic parametric space, with the main parameter being connections, secondary parameters being location, shape, and size, and tertiary parameters also helpful. An example of a neuron type is blue cone, of the neuron group cone, of the neuron class photoreceptor, of the neuron category visual, of the neuron division sensory, of the cell type neuron, with no known neuron varieties of this neuron type, at least in rat (see Bota & Swanson, 2007). Neuron types can consist of a single identified neuron (most common for invertebrates) or a set of neurons sharing the same parametric space. Neuron types were first recognized by Purkinje (1838) and first classified by Golgi (1873).