Order 1. 



HYDROIDA. 



689 



oLl? w>?^ r,! "?'" '""'"'" °'' P''"*"^"^''"^. l*"' gradually acquire the form and structure of the original ; and 



hese when they have arrived at maturity and are able to maintain their own existence, become detached and 



Jive independently. Before this separation takes place, however, but after their stomach and tentacula are fully 



developed, the digestive cavities of the 



young IlydrjB (of which several may exist at once upon one stock) : 



ronnectea with that of the parent by an aperture in their footstalks; and fluids can pass readily from one to the 



other. Now this is, in fact, the essential condition of such a com- 

 pound structure as the one represented in Fig. 12 ; for aU the polypes 

 in such a structure have been in reality produced by gemmation from 

 a single individual ; and their digestive cavities are united by tubes 

 which proceed from the base of each, along the stalk, to communicate 

 with the cavity of the central stem. There is this peculiarity, however, 

 in the compound polypes of this order— viz. that the vitality seems 

 rather to exist in the stem and branches than in the polypes seated 

 upon them ; for the polypes not unfrequently die, are cast off, 

 and then renewed, like the leaves of a tree. A circulation of fluid 

 may be seen to take place within the stem and branches of many of 

 the compound Hydroida. Like that of the Ascidians, it is reversed 

 at intervals ; the flow being sometimes very rapid, then slackening 

 and stopping, and then recommencing in the opposite direction, 

 Bometimes after an interval, sometimes immediately. 



The study of the reproduction of the Hydroida has disclosed some 

 very curious facts. Besides propagating itself by buds, in the manner 

 iust described, the Hydra, towards the approach of winter, forms 

 ovisacs in the membranous substance of its body near the foot ; 

 whilst spermatic vesicles are formed in like manner near the oral 

 extremity. These discharge their contents — ova and spermatozoa — 

 at the same time ; and from the fertilized ova it is probable that a 

 new generation of Hydras is developed. In the compound Hydroida, 

 however, we do not find either eggs or gemmaa produced from the 

 bodies of the individual polypes. For the extension of the parent 

 structure, new polype-cells and polypes are evolved from tlie stem 

 and branches ; whilst for the production of an entirely new genera- 

 tion, we find a very distinct and most remarkable provision. In 

 many of the solitary or slightly branching genera of the marine 

 Fio. 12.— Caihpanulabia. Hydroida, belonging to the family Txihulai-idcc, the body of the polype 



produces buds altogether unlike itself; these buds are, in fact, true Miduscc, and have been described as such after 

 their detachment and their attainment of their complete form. It is by the Medusa; which freely swim through 

 the water, and which thus go to form new colonies elsewhere, that the true ova are produced, which are developed 

 at first into polypes ; these polypes evolve Medusa-buds ; and from the mature Medusae, ova are again produced, 

 from which a new generation arises, to go through the same cui-ious series of phenomena. There is little diffi- 

 culty in perceiving here a close analogy with the history of vegetable development. The seed and the egg are 

 essentially the same thing ; from it spring in the one case a stem and leaves, in the other a stem and pol\ pes ; 

 these may extend by gemmation to any degree, producing new leaves or new polypes ; but after a time a different 

 set of buds appears, the flower-buds and the Medusae, containing distinct sexual organs, by which seeds and ova 

 are again generated. The only difference that even seems essential, lies in the detacAmeiit of the Medusa-buds; 

 but this is only that they may possess locomotive powers which shall carry them to a distance, in order that the 

 ova may be widely scattered through the ocean. 



In other Compound Hydroida, however, tl ere is a distinct apparatus for the development of the Medusa-buds. 

 This consists of a large cell or capsule, which «as formerly designated as an "ovarian vesicle," being supposed 

 to produce ova from which new polypes arise. But it is now known that in many cases, at least, the bodies 

 really generated in them are 3fedxisa-buds, which become detached (sometimes in a very immature form), and 

 swim forth to deposit their ova, from which a new generation of polypes will arise, in some distai.t spot. This 

 is certainly the case with the Campanvlaridce ; but whether the "gemmules" which issue from the ovarian 

 vesicles of the Sertularidm are of the same nature, has not yet been ascertained. 



Thus we have seen that the Hydraform Polypes are so closely connected with the Pulmograde Medusae, that 

 they cannot be justly separated from each other. For whilst the animals best known to us as J/eJiisa' can be 

 shown to pass the early part of their lives in the Polypoid condition, the animals best known to us as Hydraform 

 Polypei are sexually propagated by Medvsan bodies springing from them by gemmation. 

 The following is Dr. Johnston's classification of this order : — 



Section A. — Ovisacs or bulbules naked, bud-like, pullulating from the bases of the tentacula. 

 Family 1. Corynidce. Polypes naked, or with only a rudimentary polypidom. 

 Family 2. Tubularidce. Polypidom fistular ; the tentacula whorled. 

 Section B. Ovisacs in the form of horny capsules or vesicles scattered on the polypidoms, and deciduous. 

 Family 3. Sertularidce. Cells of the polypes sessile. 

 Family 5. Campanulandcc. Polype-cells on ringed stalks. 

 Bection C. Polypes propagating by buds and ova, which develope themselves on and in the body of tlie parent. 



Family 5. Bydraidte. 



2 X 



