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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    supposed to be auditory; but the conjecture is hardly borne out by an extended investigation of their structiire. At the base of the tentacles (Woodcut, fig. xii. b) there is often a collection of pigment-cells (a coloured spot or ocellus) in which a crystalline body is sometimes imbedded, as in Eleutheria and Clavatella. In Tiaropsis diademata, Agassiz describes as many as fourteen highly refractive bodies, or lenses, as forming a crescent within the pigment- spot*. These ocelli are regarded, with much probability, as rudimentary organs of vision, or at least as holding a place in the Hydroid economy analogous to that of the eye in higher organisms. It is interesting to remark that these very simple organs of sense make their appearance only in the zooids which are destined to become free. The digestive cavity is lodged in the manubrium, and the nutritive material prepared in it passes into the canals and circiilates through them; the oral extremity is some- times lobed, and sometimes furnished with tentacular appendages, which assist in the capture of food. The generative elements are developed either between the two membranes that form the walls of the manubrium, or in special sacs, which are borne on the radiating canals (Woodcut, fig. xi. o). They usually occur in the former position amongst the Athecata, and in the latter amongst the Thecaphora; but the distinction is not universal. The period at which the ovaries and spermaries make their appearance varies considerably; in some cases they are developed before the zooid detaches itself, in others not Tintil long after its liberation. The free gonozooid has by no means attained its full * Contributions to t]ie N. II. of the U. S. vol. iv. p. 309.