1. The muscle organs recently described by Alexandrowicz in the tails of Homarus vulgaris and Palinurus vulgaris , have also been found to be present in the crayfish ( Cambarus clarkii ) and the rock lobster ( Panulirus interruptus ), and a study of them in the latter animals has been undertaken. 2. The position of these units within the abdomen of the crayfish differs from their position in the lobsters. In almost all other respects, however, there is substantial correspondence in their morphological features. 3. The organs are easily isolated and it is found that stretching them is accompanied by a discharge in one of the axons supplying each unit. There are two receptors in each half-segment of the abdomen and their responses show considerable differences. One has a very low threshold and can be sustained in continuous discharge for several hours. The threshold of the other is high and this receptor usually adapts completely to the most extreme stretch in less than 1 min. 4. A phenomenon which is designated ‘over-stretch’ has been observed. All-but-maximal stretch causes a reversible cessation of the previous high-frequency discharge. The discharge can be restored by slight release of tension, but even if the tension producing ‘over-stretch’ is maintained for a sufficient length of time, discharge will resume spontaneously. The possibility that this spontaneous return is due to a slow rupture of the receptor tissues is not excluded. 5. The muscular regions of the organs are not necessary for their sensory function. 6. It has been possible to show that stimulation of the nerve trunk can induce contraction in these muscular regions and that such contraction results in discharge in the sensory axons. 7. The unit with the low stretch-threshold responds to low concentrations of ACh and this effect is potentiated by eserine. In concentrations above 10 -6 before eserinization and above 10 -8 after eserinization, ACh consistently initiates rhythmic responses in relaxed units and augments the discharge from those under tension. 8. ACh affects the sensory mechanism directly; the muscular regions of the organs are not necessary for its action. 9. Atropine inhibits the ACh excitation, but only increases the response of the receptors to stretch. Eserine, except in high concentrations, has no consistent effect on the normal stretch discharge. 10. The possibility that ACh has a role in the normal receptor mechanism is discussed, but the question is left open.