The response mechanism in ascidians

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Abstract
A method is described for investigating the reactions and spontaneous activity of two species of ascidian,Phallusia mammillataandAscidiella aspersa, which does not disturb the animals unduly. The method can be used for obtaining graphic records of the so-called ‘reflexes’ of ascidians.If due attention is paid to certain factors which affect the state ofthe animals, they can be made to respond to electrical stimuli applied to the outside of the test in a way which can be observed repeatedly in the same animal, in other animals of the same species and in at least two genera of ascidian.Electrical stimuli applied to the test affect the excitable system of the whole animal in an all-or-nothing manner.Response to graded electrical stimulation reveals a facilitation mechanism which gives enormous enhancement to the size and speed of response to the second of a close pair of effective stimuli. It is suggested that this mechanism is responsible for the production of the rapid squirt of the large sedentary ascidians.A possible cause of the very rapid fatigue of ascidians to all kinds of stimulation is fatigue of the facilitation mechanism following a period of stimulation.A parallel is drawn between the responses ofMetridiumand the responses ofPhallusiaandAscidiella. The responses have some common features and the response mechanism appears to serve a similar purpose in the life of the animals.

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