T. H. Huxley 1851, a report on the investigations of Johannes Peter Müller into the anatomy and development of echinoderms.

Huxley had concluded his description of the development of echinoderms with a discussion of ‘one of the most comprehensive and interesting zoological theories of modern times— … “the alternation of generations.” ’ (T. H. Huxley 1851, p. 13). He believed that previous work, most notably by Johannes Japetus Smith Steenstrup and Richard Owen, raised philosophical difficulties about the nature and definition of an ‘individual’ in groups such as the echinoderms that have a succession of very different forms of larvae. Huxley therefore coined the term ‘zooid’ to describe a form that has an independent existence but that is only one of several successive forms that result from the development of a single ovum. In this case, the individual consists of the sum of the zooids; for example, according to Huxley: ‘Instead of saying then, that in a given species, there is an alternation of so many generations, we should say that the individual consists of so many zooids’ ( p. 15). See Winsor 1976, p. 65, and Farley 1982.