Q 1. Hydra can move from one place to another by flexing the body and gripping the substrate with the tentacles and then looping the body over the mouth to gain a new position. Explain exactly what mechanisms and principles are involved in this movement? The full answer to this question involves the hydrostatic skeleton formed by the gastrovascular cavity and contractile cells. [5-6 sentences]
The movement of hydra is called looping/somersaulting. This movement is possible due to the presence of a hydrostatic skeleton. This fluid filled chamber can be bent and moulded into different shapes. When the muscle cells (longitudinal and circular) contract, hydrostatic fluid from the gastro vascular cavity (GVC) is squeezed in a certain direction (depending on type of muscle cell). The contraction of longitudinal muscles cells leads to hydrostatic fluid from the GVC in an outwards and downwards direction, this makes the hydra seem compressed. When the circular muscle cells contract (squeezing the GVC from the sides) the hydrostatic liquid is forced in an upwards manner, this in turn elongates the hydra. A combination of elongation and compression allows itself to somersault itself to a new position.
Q 2. What reasons can you think of that might limit this type of body form to aquatic systems? [5-6 sentences]
Being a freshwater dwelling organism, the obvious source of the hydrostatic fluid in the gastro vascular cavity for the hydra is from its surroundings