In continuous development, the partially independent beings which are produced by gemmation or fission from the primitive organism, to which they remain permanently attached, are termed "zooids." In other words, "zooids" are the more or less individualised members of which the Hydroid colony is made up.

In discontinuous development, where certain portions of the "individual " are separated as completely independent beings, these detached portions are likewise termed "zooids ;" that which is first formed being distinguished as the "producing zooid," whilst that which separates from it is known as the "produced zooid." In a great number of Hydrozoa there exist two distinct sets of zooids, one of which is destined for the nutrition of the colony, and has nothing to do with generation, whilst the functions of the other, as far as the colony is concerned, are wholly reproductive. For the whole assemblage of the nutritive zooids of a Hydrozoon Professor Allman has proposed the term "trophosome," applying the term "gonosome" to the entire assemblage of the reproductive zooids. In such Hydrozoa, therefore, as possess these two distinct sets of zooids, the "individual," zoologically speaking, is composed of a trophosome and a gonosome. It follows from this that neither the trophosome nor the gonosome, however apparently independent, and though endowed with intrinsic powers of nutrition and locomotion, can be looked upon as an "individual," in the scientific sense of this term. As a rule, the zooids of the trophosome are all like one another, or are "homomorphic" but there are some cases (as in Hydractinia, and in the nematophores of the Plumularidoe) in which some of zooids of the trophosome are unlike the others. The zooids of the gonosome, on the other hand, are normally unlike, or are "hetero-morphic," consisting of two or three different sets of zooids, each with its special duty in the generative functions of the Hydroid colony.