I'm not a biologist, but I do read that a zooid is a single cell that can move independently within a larger organism.    From Wordnik.com.  [Angola, Spare Standards And A Zooid: New Jazz Albums] Reference

It is not easy to imagine two objects more widely different in appearance than a bristle or vibraculum, and an avicularium like the head of a bird; yet they are almost certainly homologous and have been developed from the same common source, namely a zooid with its cell.    From Wordnik.com.  [VII. Miscellaneous Objections to the Theory of Natural Selection] Reference

Mr. Busk, however, does not know of any gradations now existing between a zooid and an avicularium.    From Wordnik.com.  [VII. Miscellaneous Objections to the Theory of Natural Selection] Reference

Such zooids are specialised to such an extent that they lack the structures associated with other functions and are therefore dependent for survival on the others to do what the particular zooid cannot do by itself.    From Wordnik.com.  [Pounding The Rock] Reference

The vibracula may have been directly developed from the lips of the cells, without having passed through the avicularian stage; but it seems more probable that they have passed through this stage, as during the early stages of the transformation, the other parts of the cell with the included zooid could hardly have disappeared at once.    From Wordnik.com.  [VII. Miscellaneous Objections to the Theory of Natural Selection] Reference

It is interesting to see two such widely different organs developed from a common origin; and as the moveable lip of the cell serves as a protection to the zooid, there is no difficulty in believing that all the gradations, by which the lip became converted first into the lower mandible of an avicularium and then into an elongated bristle, likewise served as a protection in different ways and under different circumstances.    From Wordnik.com.  [VII. Miscellaneous Objections to the Theory of Natural Selection] Reference

1 Each such zooid in these pelagic colonial hydroids or hydrozoans has a high degree of specialization and, although structurally similar to other solitary animals, are all attached to each other and physiologically integrated rather than living independently.    From Wordnik.com.  [Pounding The Rock] Reference