[Illustration: FIG. 19.--Diagram showing method of branching in the _Aglaophenia_-type. Polyp 7 has produced as its first bud, 8; as its second bud, a^7, which starts a uniserial pinnule; and as a third bud I^7, which starts a biserial branch (II^7-VI^7) that repeats the structure of the main stem and gives off pinnules. The main stem is indicated by-·-·-·, the new stem by ······.] Besides the three types of individual above mentioned, there are other appendages of hydroid colonies, of which the individuality is doubtful. Such are the "guard-polyps" (machopolyps) of _Plumularidae_, which are often regarded as individuals of the nature of dactylozoids, but from a study of the mode of budding in this hydroid family Driesch concluded that the guard-polyps were not true polyp-individuals, although each is enclosed in a small protecting cup of the perisarc, known as a nematophore. Again, the spines arising from the basal crust of _Podocoryne_ have been interpreted by some authors as reduced polyps. 3. _The Medusa._--In the Hydromedusae the medusa-individual occurs, as already stated, in one of two conditions, either as an independent organism leading a true life in the open seas, or as a subordinate individuality in the hydroid colony, from which it is never set free; it then becomes a mere reproductive appendage or _gonophore_, losing successively its organs of sense, locomotion and nutrition, until its medusoid nature and organization become scarcely recognizable. Hence it is convenient to consider the morphology of the medusa from these two aspects. (a) _The Medusa as an Independent Organism._--The general structure and characteristics of the medusa are described elsewhere (see articles HYDROZOA and MEDUSA), and it is only necessary here to deal with the peculiarities of the Hydromedusa. [Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of the Council of the Ray Society. FIG. 20.--_Cladonema radiatum_, the medusa walking on the basal branches of its tentacles (t), which are turned up over the body.] [Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of the Council of the Ray Society. FIG. 21.--_Clavatella prolifera_, ambulatory medusa. t, tentacles; oc, ocelli.]