1. In the abdomen and thorax of some groups of Crustacea (Stomatopoda, Decapoda Macrura, and Anomura) ganglion cells have been found with ramifications into special muscle-fibres. It is assumed that these are organs for response to stimuli resulting from muscular activity and therefore the name ‘muscle receptor organs’ has been adopted for them. Each muscle receptor unit consists of (a) a thin muscle, (b) one ganglion cell connected with this muscle by means of numerous dendritic processes, and (c) various nerves supplying the muscle and entering into connexion with ganglion cells.

This paper describes the results of a study of these organs in the abdomen of Homarus vulgaris and Palinurus vulgaris.

2. In each of the six abdominal segments of these animals there are two muscle receptor units on each side lying close to one another at the level of the superficial dorsal muscles. Their muscle components are quite distinct from the neighbouring muscles and preserve their individuality throughout their whole course and at their attachments. Moreover, the two muscles of the same side exhibit differences in their length, their attachments, and even their histological structure. Each muscle in about the middle of its length has a region made up of connective tissue fibres which may be regarded as an intercalated tendon.

3. Situated near to and in connexion with each of these muscle units is one large nerve-cell; there are, therefore, four such cells in each segment and a total of twenty-four in the abdomen. The cells are multipolar in shape with a variable number of short dendritic processes abundantly ramifying in the intercalated tendinous region of the muscle. The long processes, the axons, join the dorsal branch of the nerve supplying the extensor muscles and run in it towards the ganglionic cord.

4. In preparations made from embryonic lobsters it has been possible to establish that these axons bifurcate after entering the ganglionic cord, and the resulting branches run in opposite directions. Associating with similar fibres from other segments they form a tract situated in the nerve-cord near to its median line and running through all the ganglia of the abdominal and thoracic segments.

5. It has been found that in addition to the ganglion cells, at least three kinds of nerves take part in the innervation of the muscle receptors. They have been described under the names of: (a) motor nerves, (b) thick accessory nerve, and (c) thin accessory nerve.

6. Special means for protecting the muscle receptor organs are present. The nerve-cells are encapsuled and encircled by several layers of thin membranous tissue. The muscles are surrounded by connective tissue fibres and a special arrangement of these fibres supports the muscles in position.

7. As regards the function of these organs, the hypothesis is put forward that they might come into action during vigorous movements of the abdomen in the escape reaction of the animal. If this be so, they may perhaps convey inhibitory impulsesto the elements causing the rapid contractions of the flexor muscles. As these contractions are governed by the giant fibre system it might be expected that the neurons of the receptor organs enter into relation with some elements of that system.

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