Figure 2.

Spontaneous contractile behaviour of Hydra. (a) Sequence of pictures demonstrating the spontaneous contraction of the body column, hydra’s most common behaviour. (b) Spontaneous contractions assessment. The contractile behaviour was described in terms of contraction frequency, defined as the number of full body contractions (MIN) occurred in one hour (red arrows), the time interval between two consecutive contractions Δt, the stretching capacity S_max, which is the maximum body elongation (MAX) achieved after a contraction and the contractile capacity ΔS, the difference in a polyp’s shape between MAX and MIN states.