RADIATA. 



701 



u 



I lijf 



o, its bony as httie knoos or protuberan.es, o-t gr.du.uy acquire the form and structure of the original • and 

 Uiese,wkenthej haye amved at maturity and .re able to maintain their own existence, become detached and 

 live independently. Before this separation takes place, however, but after their Btomach and tentaeula are fully 

 dereloped, the digestive cavities of the young Hydra, (of which several may exist at once upon one stock) are 



connected mth that of the parent by an aperture in their footstalks; and fluids can pas, readily from une 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 all 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, howevi r, 

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

 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 Ilydroida. Like that of the AscidLins, it is reversed 

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

 and stopping, and then recommencing in the opposite direction, 

 sometimes after an interval, sometimes immediately. 



The study of the reproduction of the Ilydroida has disclosed some 

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

 just 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 Hydra is developed. In the compound Ilydroida, 

 however, we do not find cither eggs or gemma? produced from the 

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

 structure, new polype-cells and polypes are evolved from the 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 

 Fig. 12.— Campanplaria. Hydroida, belonging to the family Tubularidce, the bodj of the polype 



produces buds altogether unlike itself; these buds are. in fact, true Medus"; and have been described as such aft. r 

 their detachment and their attainment of their complete form. It is by the Medu.su: which freely swim through 

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

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

 fi ..in which a new generation arises, to go through the same curious series of phenom e na. 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 Btem and poljpes ; 

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

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

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



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

 ova may be widely scatter. d through the ocean. 



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



This consists of a large cell or capsule, which was formerly designated as an " ovarian reside," bi in- sup] 1 



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

 really generated in them are Meduso-otidt, which become .1. ta> h. .1 -..inctiinesin a very immature form), and 



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

 is certainly the ease with the r.im/...i.i/J.iri./o ; but whether the " gcminul. s " which issue from the 01 Irian 

 vesicles of the SertularidcB are of the same nature, has not yet 1" • u ascertained. 



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

 they cannot be justly separated from each Ot.ier. For whilst the animals best known to us :.- Uedusa cm he 

 shown to pass the early part of their lives in the Polypoid condition, the animals best known to nsas.fi] . 



■Polypi:-* arc sexually propagated bj I/. ./■■■■ M bo.li.-s springing from them by gemmation. 



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



Section A. — Ovisacs ..r bulimies naked, bud-like, pullulating from th.- ba fthe tentaeula. 



Family 1. CorynidcB. Polypes naked, or with only s rudlmentarj polypidom. 



Famil] -'. I 'u'aridr I'olypidom hstular ; the tentaeula whorlc.l. 



Section It. Ovisacs in the form of horny capsuli Battered on the polypidoms, and deciduous. 



Family 3. Sertitliri /.»■. Cells of tin polypes s — lie. 



Famiij i lartda. Polype-oells on ringed stalks. 



Section 0. Polyp s propagating bj buds and ova, which develops tl on and In the bodj of the parent. 



Family ■'>. Jly-lr.itd ' i . 



