[Illustration: Fig. 100. Female colony of Hydractinia; _a_ sterile individual, _b_ fertile individual producing female Medusæ, _c_ fertile individual with globular tentacles without Medusæ, _d_ _e_ _f_ _g_ _h_ _i_ Medusæ in different stages of growth, _o_ mouth tentacles. (_Agassiz_.)]
[Illustration: Fig. 101. Male colony; _a_ _a_ sterile individuals, _b_ fertile individuals producing male Medusæ, _d_; _o_ globular tentacles, _t_ slender tentacles of sterile individual. (_Agassiz_.)]
Among our Medusæ derived from a Tubularian stock is the Hybocodon, viz. the hunchbacked Medusa (Fig. 102), a singular little Jelly-fish, odd and unsymmetrical in shape, as its name indicates, and interesting from its relations to one of our floating communities, the Nanomia, presently to be described. Instead of the evenly proportioned bell of the ordinary Medusæ, the Hybocodon has a one-sided outline (Fig. 102), one large tentacle only being fully developed, while the others remain always abortive, so that the whole weight of the structure is thrown on one half of the bell. Upon this large tentacle small Jelly-fishes, similar to the original, are produced by budding, this process going on till ten or twelve such Jelly-fishes (Fig. 103) may be seen suspended from the tentacle. Up to this time it has remained connected with the Hydroid from which it arises, a rather large Tubularian, usually growing singly (Fig. 104), and of a deep orange-red in color. But at this stage of its existence it frees itself, and leads an independent life hereafter, swimming about with a quick, darting motion. In the account of the Nanomia, the homology between its scale, or abortive Medusa, and the Hybocodon, is traced in detail, and I need only allude to it here. Though this Medusa is so peculiar in appearance, the Tubularian from which it is derived is very like the _Tubularia Couthouyi_, already described. This is one of the instances before alluded to, in which closely allied forms give rise to very dissimilar ones, or, as in many cases, the very reverse of this takes place, and closely allied forms arise from very dissimilar ones.
[Illustration: Fig. 102. Unsymmetrical free Medusa of Hybocodon; _r_ _o_ chymiferous tubes, _v_ proboscis, _s_ circular tube, _m_ young Medusæ at base of long tentacle _t_. (_Agassiz_.)]