The nervous system of an earthworm is composed of a primitive brain of fused ganglia, a ventral nerve cord, and peripheral nerves. Impulses from the earthworm’s sensory cells are transmitted by the peripheral nerves to certain parts of the body and proper responsive movements are coordinated. If sensory cells indicate that the area outside its burrow is too hot or too cold, for instance, the earthworm will readjust its position so that its less vital, posterior portion is facing that direction. When the organism senses that conditions outside are favorable, it resides with its head towards the surface.