Volume 1

A history of the British hydroid zoophytes / by Thomas Hincks.

Date:
1868
Catalogue details

Licence: Public Domain Mark

Credit: A history of the British hydroid zoophytes / by Thomas Hincks. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    development and perfect form on leaving the parent stock; in a large proportion of cases it undergoes very con- siderable change subsequently. The form of the umbrella may alter^ and the marginal tentacles and other bodies and even the radiating canals increase greatly in number; while the manubrium may become much elongated, or develope additional oral apjiendages *. The early and mature states are often so dissimilar as to have been re- ferred to different species; and as there is seldom the opportunity of observing the whole course of development, the varying phases of the sexual zooid are a source of much perplexity to the systematist f. Gemmation is not confined to the fixed portions of the Hydroid colony ; it also enters into the history of the free and locomotive zooids. In many cases they manifest the vegetative tendencies of their tribe, and multiply rapidly by budding. Gemmation seems usually to take place when the true reproductive function is in abeyance. Thus in the spring the gonozooid of Clavatella developes buds on the margin of the body between each pair of tentacles, which are cast off at a certain stage of growth; while later on in the year the vegetative activity ceases, and reproduction by ova and spermatozoa takes its place. These buds, which are analogous to those produced in such profusion by the Hydra, bear an exact resemblance, when mature, to the zooid that originated them. In other cases they spring from the manubrium, or from the bul- * A. Agassiz has pointed out that the tentacles are developed in a certain fixed order, and has given the formula of development for many species. (Proc. Boston Nat. Hist. Soc. vol. ix., August 1862.) + A good illustration of the changes which the detached zooid may undergo before reaching maturity, and of the eom]3lexity of structure which it may finally attain, is afforded by the genera Bougainvilliu and Zygodactyla.
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