are living creatures which works with another creature, and depends on that creature for its life.

are species living with a cooperating host species in a relationship that is mutually beneficial. References Campbell, R.D. 1987. A new species of Hydra (Cnidaria: Hydrozoa) from North America with comments on species clusters within the genus. Zoological Journal of the Linnean Society 91. Lenhoff, H.M., 1983. Hydra: Research Methods. Plenum Press, New York. Thorp, J.T. and Kovich, A.P. 1991. Ecology and Classification of North American Freshwater Invertebrates. Academic Press, San Diego.

organisms of two different species that exist in a close interrelationship that is benfical to both.