Hypostome

This mode of attachment and support of the hypostome in proparian trilobites is also shown by Öpik's work (1938, and references) and by Whittington (1941b, p. 514-517) and Evitt and Whittington (1953).

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2012, Georgia E. Lesh-Laurie, Chapter 5. Expression and Maintenance of Organismic Polarity, Allison L. Burnett (editor), Biology of Hydra, Harcourt Brace Jovanovich (Academic Press), pages 160-161, As a result or these interactions, the presence of a hypostome inhibits the formation of another hypostome nearby. Removal of the hypostome causes the level of inhibition to fall; regeneration of it restores this level (Webster, 1966).

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The polyps of the two observed populations of Alatina cf. moseri showed no distinct differences.[…]The body of the polyp is divided into three parts: the hypostome, the calyx and a stalk region. The latter includes a basal disc and a tiny periderm beaker enveloping the pedal region. The hypostome of polyps of both populations was four-lipped (Fig. 5.33F and G), balloon-shaped and ~14%–15% of the total body length (TBL). It was not completely contractible into the body.

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Figure 7.4 Argas persicus^([fowl tick])[…](b) Hypostome of larval stage. The toothed hypostome is easily visible in the larval stage.

Source: wiktionary