Hydrocaulus, and forms a cup, the hydrangium or hydrotheca (h, t), standing off from the body, into which the hydranth can be retracted for shelter and protection.
Amphibrachium grows in the tissues of a sponge, Euplectella, and protrudes its hydranth into the canal-system of the sponge; and Lar grows on the tubes of the worm Sabella.
The hydranth almost always has a single circlet of tentacles, like the Bougainvillea-type in the preceding sub-order; an exception is the curious genus Clathrozoon, in which the hydranth has a single tentacle.
The gymnoblastic polyp usually has a distinct perisarc investing the hydrorhiza and the hydrocaulus, sometimes also the hydranth as far as the bases of the tentacles (Bimeria); but in such cases the perisarc forms a closely-fitting investment or cuticule on the hydranth, never a hydrotheca standing off from it, as in the next sub-order.
In this sub-order the characters of the hydranth are very variable, probably owing to the fact that it is exposed and not protected by a hydrotheca,.
On the upper face of the hydranth the crown of tentacles (t) surrounds the peristome, from which rises the conical hypostome, bearing the mouth at its extremity.
In both cases the hydranth is extremely reduced and has no tentacles, and the polyp forms a colony by budding from the base.
Thus the affinities of the hydranth are clearly, as Dendy points out, After Haeckel, System der Medusen, by permission of Gustav Fischer.
Speaking generally, three principal types of hydranth can be distinguished, each with subordinate varieties of form.
The body, representing the hydranth of an ordinary hydroid, has the aboral portion modified into a float, from which hangs down a proboscis bearing the mouth.