HYDROZOA 



63 



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" 



Fio. IS. Diagrams to exhibit the plan of structure of hydriform and medusiform 

 persons (all eicept 5 are vertical sections). A, base of tentacles, margin of the 

 dice; B, oral margin; Ma, mannbrium; Te, tentacle; CV, circular vessel; 

 EnL, endodenn lamella ; ot, otocyst ; <x, ocellus olf, olfactory pit ; H, hood of 

 tentaculocyst ; mg. genitalia developing in manubrinm; dg, genitalia develop- 

 ing in the disc (wall of a radiating canal) ; GP, sub-genital pits of the sub- 

 umbrella; Gf, gastrai filaments; re. velum. 1, Form Intermediate between 

 medusa-form and hydra-form. 2, Hydra-form with wide disc, manubrinm, 

 and solid tentacles (Tubularian). 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 right; a velum, two 

 possible positions of the genitalia, and two kinds of sense-organs are shown 

 (Hydromedtace). 5, A similar medusa-form seen from the surface. 6, Section 

 of Aurtlia attrita, to show especially the nature of the sub-genital pits, GP. 

 outside the genital frills, and the position of the gastrai filaments GF, 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 

 Comatula 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 



" strobilation " 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 gastrai 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 larva?. 



The second line of descent which has given rise to those 

 Hydrozoa, known as Hydromedusce not only acquired at 

 the start a different mode of producing medusiform persons, 

 but the medusiform persons acquired characters differing 

 from those of the Scyptomediuce 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 Hydromedufce 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 medusae 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 Hydromtdusce series ever 

 have gastrai 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 (Eudendrium), Campanvlaria, 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.) The hydriform stage was suppressed altogether, 

 and, as in some Scyphomedusce, so here too the diblastula 

 developed directly into a medusa (Trachomedusce, Narco- 

 mfduste, and probably some Leptomedusce like Thaumanliat 

 and JZyuorea, and some Anthomedusce like Oceania, and 

 Turritopsis). 



