. The animal kingdom : arranged after its organization; forming a natural history of animals, and an introduction to comparative anatomy. Zoology. Order 1. HYDROIDA. r'?^ r^' "I"' ^"""^^ '"? P''°»"»^"^°^««. but gradually acquire the form and structure of the onginal ; and these when they have arrived at maturity and are able to maintain their own existence, become detached and ^Zln r!f ",^^' *""' separation takes place, however, but after their stomach and tontacula are fully tZnZi l.'f T "''""'' °^ '^' ^°""^ "
. The animal kingdom : arranged after its organization; forming a natural history of animals, and an introduction to comparative anatomy. Zoology. Order 1. HYDROIDA. r'?^ r^' "I"' ^"""^^ '"? P''°»"»^"^°^««. but gradually acquire the form and structure of the onginal ; and these when they have arrived at maturity and are able to maintain their own existence, become detached and ^Zln r!f ",^^' *""' separation takes place, however, but after their stomach and tontacula are fully tZnZi l.'f T "''""'' °^ '^' ^°""^ "^^^* '°^ ^'"'='' ^^^"'" ""'^y «-^i^t «' «?»=« "Pon o"*^ stock) are 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 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, 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, towa
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Keywords: ., bookauthorwe, bookcentury1800, bookdecade1860, booksubjectzoology
