The zooids of the Gymnoblastea present considerable diversity of form and structure. The tentacles may be reduced to one (in _Monobrachium_) or two (in _Lar sabellarum_), but usually the number is variable in each individual colony. In many cases, such as _Cordylophora_, _Clava_, and many others, the tentacles are irregularly scattered on the sides of the zooids. In others there may be a single circlet of about ten or twelve tentacles round the base of the hypostome. In some genera the tentacles are arranged in two series (_Tubularia_, _Corymorpha_, _Monocaulus_), a distal series round the margin of the mouth which may be arranged in a single circlet or scattered irregularly on the hypostome, and a proximal series arranged in a single circlet some little distance from the mouth. In _Branchiocerianthus imperator_ the number of tentacles is very great, each of the two circlets consisting of about two hundred tentacles. [Illustration: FIG. 130.—Diagrammatic sketch to show the method of branching of _Bougainvillia_. _gon_, Gonophores; _Hr_, hydrorhiza; _t.z_, terminal zooid.] The zooids of the hydrosome are usually monomorphic, but there are cases in which different forms of zooid occur in the same colony. In _Hydractinia_, for example, no less than four different kinds of zooids have been described. These are called gastrozooids, dactylozooids, tentaculozooids, and blastostyles respectively. The "gastrozooids" are provided with a conical hypostome bearing the mouth and two closely-set circlets of some ten to thirty tentacles. The "dactylozooids" are longer than the gastrozooids and have the habit of actively coiling and {265}uncoiling themselves; they have a small mouth and a single circlet of rudimentary tentacles. The "tentaculozooids" are situated at the outskirts of the colony, and are very long and slender, with rudimentary tentacles and no mouth. The "blastostyles," usually shorter than the gastrozooids, have two circlets of rudimentary tentacles and a mouth. They bear on their sides the spherical or oval gonophores.