For instance, among the Hydroida, in the great majority of cases, the egg produces a body more or less resembling the common _Hydra_ of our ponds, and known technically as the “trophosome,” which develops into the well-known Medusæ or jelly-fishes. The group, however, for which Prof. Allman has proposed the term Monopsea,[25] and of which the genus _Ægina_ may be taken as the type, is, as he says, distinguished by the absence of a hydriform stage, “the ovum becoming developed through direct metamorphosis into a medusiform body, just as in the other orders it is developed into a hydriform body.” Fig. 34 represents, after Allman, a colony of _Bougainvillea fruticosa_ of the natural size. It is a British species, which is found growing on buoys, floating timber, &c., and, says Allman,[26] “when in health and vigour, offers a spectacle unsurpassed in interest by any other species—every branchlet crowned by its graceful hydranth and budding with Medusæ in all stages of development (Fig. 35), some still in the condition of minute buds, in which no trace of the definite Medusa-form can yet be detected; others, in which the outlines of the Medusa can be distinctly traced within the transparent _ectothèque_ (external layer); others, again, just casting off this thin outer pellicle, and others completely freed from it, struggling with convulsive efforts to break loose from the colony, and finally launched forth in the full enjoyment of their freedom into the surrounding water. I know of no form in which so many of the characteristic features of a typical hydroid are more finely expressed than in this beautiful species.” [Illustration: FIG. 35.—Portion of colony of _Bougainvillea fruticosa_, more magnified.] [Illustration: FIG. 36.—The Medusa form of the same species.] Fig. 36 represents the Medusa form of this species, and the development thus described may be regarded as typical of the Hydroida; yet, as already mentioned, the Æginidæ do not present us with any stage corresponding to the fixed condition of _Bougainvillea_, but, on the contrary, are developed into Medusæ direct from the egg. On the other hand, there are groups in which the Medusiform stage becomes less and less important. [Illustration: FIG. 37, Larva of Prawn, Nauplius stage (after F. Müller). 38, Larva of Prawn, more advanced, Zoëa stage.]