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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    The capitate tentacle^ of which we have good examples iu Coryne aud Clavatella, bears on its summit a globular head, consisting of a collection of thread-cells—a formi- dable battery of offensive weapons, which is brought to bear on any passing prey. The arm is also endowed with vigo- rous percussive power, and when its numerous poisoned threads are brought into play, it can hardly fail to arrest and paralyze any of the smaller creatures that may come within its range. (Plate VII. fig. 1 b.) The thread-cells, which bear so important a part in the Hydroid economy, exhibit many modifications. They occur in the ectodermal layer, and are present in as- tonishing profusion, not only on the tentacles, but in other portions of the structure. They consist of minute sacs imbedded in the flesh and fiUed with fluid, which contain a long and delicate thread, capable of being projected with considerable force and inconceivable rapidity. These threads buiy themselves in any soft substance against which they may be directed, and, it is supposed, convey into the wound which they make some poisonous fluid. The thread-cell is a most interesting piece of structure. The long dart which it encloses is borne on a continuation of the inner wall of the sac (the sheath of some writers, the axial body of others), which is often covered with barbs. (Woodcut, fig. iii. c.) When retracted, the thread is spirally coiled within the cell and sometimes wound round the sheath. (Woodcut, fig. iii. c.) Two kinds of thread- cell are often met with on the same species. Besides the formidable instruments with which the tentacles are armed, large bean-shaped cells are sometimes crowded together in immense quantities, as, for example, in the ectoderm of the coeuosarc in Hydranthea, and in the outer covering of