Gastrotrichs develop to a particular number of cells; further growth comes only from an increase in cell size. As is typical for such small animals, there are no respiratory or circulatory organs. Nitrogenous waste is probably excreted through the body wall, as part of respiration. The nervous system is relatively simple. The brain consists of two ganglia, one on either side of the pharynx. Each ganglion gives rise to a single nerve cord, which runs the length of the body and includes further, smaller ganglia. The main sense organs are the bristles and ciliated tufts of the body surface. Some gastrotrichs also possess light sensitive cells in the brain that work as primitive ocelli (eye spots).