The first step towards the formation of a mixed hydroid colony is undoubtedly a hastening of the sexual maturity of the medusa-individual. Normally the medusae are liberated in quite an immature state; they swim away, feed, grow and become adult mature individuals. From the bionomical point of view, the medusa is to be considered as a means of spreading the species, supplementing the deficiencies of the sessile polyp. It may be, however, that increased reproductiveness becomes of greater importance to the species than wide diffusion; such a condition will be brought about if the medusae mature quickly and are either set free in a mature condition or remain in the shelter of the polyp-colony, protected from risks of a free life in the open sea. In this way the medusa sinks from an independent personality to an organ of the polyp-colony, becoming a so-called _medusoid gonophore_, or bearer of the reproductive organs, and losing gradually all organs necessary for an independent existence, namely those of sense, locomotion and nutrition. In some cases both free medusae and gonophores may be produced from the same hydroid colony. This is the case in _Syncoryne mirabilis_ (Allman [1], p. 278) and in _Campanularia volubilis_; in the latter, free medusae are produced in summer, gonophores in winter (Duplessis [14]). Again in _Pennaria_, the male medusae are set free in a state of maturity, and have ocelli; the female medusae remain attached and have no sense organs. [Illustration: Modified from Weismann, _Entstehung der Sexualzellen bei den Hydromedusen_. FIG. 41.--Diagrams of the Structure of the Gonophores of various Hydromedusae, based on the figures of G. J. Allman and A. Weismann. A, "Meconidium" of _Gonothyraea_. B, Type of _Tubularia_. C, Type of _Garveia_, &c. D, Type of _Plumularia_, _Agalma_, &c. E, Type of _Coryne_, _Forskalia_, &c. F, G, H, Sporosacs. F, With simple spadix. G, With spadix prolonged (_Eudendrium_). H, With spadix branched (_Cordylophora_). s.c, Sub-umbral cavity. t, Tentacles. c.c, Circular canal, g, Gonads. sp, Spadix. e.l, Endoderm-lamella. ex, Ex-umbral ectoderm. ect, Ectotheca.]