Muscular and hydrostatic action in the sea-anemone, Metridium senile (L

The majority of sea-anemones present to the eye an almost constant shape which only changes as the result of activities of a very simple kind. This is true, for instance, of Calliactis parasitica, the subject of an earlier study (Pantin, 1935). The unstimu-lated animal rarely departs from the form of a short cylinder surmounted by a disk bearing the tentacles round its edge and the mouth at its centre. In contrast with Calliactis, Metridium senile is notable for the variety of shapes which it assumes (PL 7). It may be fully expanded with a narrow or with a wide column (e, / ). The same anemone may next day be found contracted down to a mere button-like object (d). It may indulge in a variety of contortions in which it is laterally bent, or possesses deep transverse constrictions locally or completely embracing the column (g, i). The whole anemone may on occasion temporarily assume a shrivelled and contracted condition (h), in which it might easily be discarded from the aquarium as dying; but which, in fact, is a normal temporary state from which there is com-paratively rapid recovery (i, j). At other times, particularly during locomotion, a part of the body may become greatly distended (c). Prolonged observation o