HYDROZOA 553 became changed in the course of the ontogeny (develop ment of the individual) into the umbrella or disc-like form, with coalesced enteric walls and radial and circular surviving spaces (medusa-form). And now the ancestry took two distinct lines, which have given rise respectively to the two great groups into which the Hydrozoa are divi sible the Scyphomedusce and the Hydromedusce. In the one set the hydriform persons of a colony, instead of each becoming metamorphosed into a medusiform person, pro ceeded each to break up into a series of transverse divisions ; each division became a medusiform person, and was liberated in its turn as a free swimming organism (figs. 26 and 27). We must suppose that this process began historically by the outgrowth of new tentacles around the point where the disc of a person fully transformed from the Flo. 16. Diagrams to exhibit the plan of structure of hydriform and medusiform persons (all except 5 are vertical sections). A, base of tentacles, margin of the diec; B, oral margin; Mn, manubihim; Tn, tentacle; CV, circular vessel; EnL, endoderm lamella; ot, otocyst ; oi ocellus olf, olfactory pit; //, hood of tentaculocyst; mg, genitulia developing in m.inubrium; <i<j, genitalia develop ing in the disc (wall of a radiating canal); Gl>, sub-genital pits of the sub- umbrella; Ol 1 , gastral filaments; Ve, velum. 1, Form intermediate between medusa-form and hydra-form. 2, Hydra-form with wide disc, maiiubrium, and solid tentacles (Titbularian). 3, Hydra-form with narrower disc, and hollow tentacles (Hydra). 4, Medusa-form with endoderm lamella on the left, the section passing through a radiating canal on the light; a velum, two possible positions of the genitalia, and two kinds of sense-organs are shown (Hydromedusir). 5, A similar medusa-form seen from the surface. 6, Section of Aurelia aitrita, to show especially the nature of the sub-genital pits, Gl outside the genital frills, and the position of the gastral filaments G / , as well as the flattened form of the disc. hydriform to the medusiform phase was loosened in its attachment and about to separate from the colony. The "hastening of events," a well-known feature of organic growth-sequences, would complete the development of the newly sprouting person before the loosened medusa had got well away, and so on with a third, fourth, and even with twenty such successive buds. The separation of the adult form from its fixed larva by fission has been justly compared by Louis Agassiz to the separation of the Comatala from its pentacrinoid larval stalk. If the stalk could only produce new Comatulce^ the analogy would be complete. Lucernaria is in the same way comparable with the stalked crinoids, being an adult form which retains the characters exhibited by the immature phases of its congeners. The Scyphomedusce do not, however, all exhibit a hydriform phase, and a production of medusae by the " sfcrobilation " or " metamerizing " of a scyphistoma. Some of them (Pelagia) " hasten events " so far that the diblastula never fixes itself, but becomes at once a single medusa, the hydriform phase of the ontogeny being alto gether omitted. Certain peculiarities of the medusa s struc ture, above all the possession of gastral filaments (solid filaments like tentacles projecting in four interradial groups near the genitalia into the enteric cavity), serve to unite Pelagia, which has no larval stage, and Lucernaria (which is always of intermediate character between hydra-form and medusa-form) with the numerous species which develop by the strobilation of hydriform larvae. The second line of descent which has given rise to those Hydrozoa known as HydromeduscB not only acquired at the start a different mode of producing medusiform persons, but the medusiform persons acquired characters differing from those of the Scyphomedusce in important (but not fundamental) features. The larval stage in this series developed the property of budding to a very great degree, so as often to form fixed tree-like colonies of considerable size. Then the transformation of the identical colony- forming persons into free-swimming persons was finally and definitively abandoned, and only a late-appearing set of buds proceeded to complete the typical changes and to become medusae. The earlier-produced buds were thus arrested in development, and became specially modified for the purposes of a fixed life as members of a colony. Thus they acquired the elongate form and the sporadic position of the tentacles which we see in some hydriform persons of the Hydromedusce group (figs. 38 and 40), and were adapted to nutrition solely (hence the term trophosome applied by Allman to such colonies). The characters of the mature generative person, with its power of detachment and free locomotion, being confined to the later buds borne on the sides of the hydriform persons or on special portions of the colony, we find that the former became more and more specialized as sexual medusiform persons in proportion as the latter became specialized as asexual hydriform persons, and thus it is that we have the remarkable phenomenon of hydriform colonies, developed from the eggs of medusae, producing as it were crops of medusae (figs. 34 and 37) which detach themselves and swim away to deposit their eggs (alternation of generations). The Hydromedusce never produce medusas by strobilation or transverse division of a hydriform person, although in rare cases the cicatrix left by a detached medusa-bud has been observed to sprout and produce a hydriform person. Neither medusiform nor hydriform persons of the Hydromedusce series ever have gastral filaments (unless they are represented by the "villi" of the Siphonophora described by Huxley, Oceanic Hydrozoa), whilst the medusa-forms always possess a velum and a comparatively simple set (four, six, or eight) of radi ating canals in the disc, the remains of the enteric lumen. The complete differentiation of hydriform and medusi form persons existing on one and the same colony having been attained in the Hydromedusce, further changes of a most remarkable character were brought about in some of the descendants of these forms. The condition which we have so far noted is perpetuated at the present day in Bougainvillia (Eudendriuni), Campamdaria, and a vast number of the so-called hydroid polyps; others have undergone further adaptational changes. We have to notice at least four important additional modifications independent of one another. (1.) "She hydriform stage was suppressed altogether, and, as in some Scyphomedusa;, so here too the diblastula developed directly into a medusa (Trackomedusce, Narco- medusce, and probably some Leptomedusce like Thaumantias and jEquorea, and some Anthomedusce like Oceania and Turritopsis).
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