A more or less elaborate system of polymorphism is of frequent occurrence in certain groups of colonial Coelenterata. For example, in many Hydroids certain individuals perform the nutritive functions for the colony, others are specialised for reproductive purposes, and so on. The variously modified individuals are connected together by a general coenenchyma, which enables the nutriment elaborated by the gastrozooids to be utilised by other members of the colony. Perhaps some of the most interesting and complex cases of polymorphism are to be found amongst the Siphonophora