Innervation of the receptors present at the various joints of the pereiopods and third maxilliped of Homarus gammarus (L.) and other macruran decapods (crustacea)

This paper gives a full account of the number and structure of the chordotonal organs present at all joints between the coxopodite and dactylopodite of the pereiopods and 3rd maxilliped of the macruran Homarus gammarus L. (H. vulgaris M. Ed.). Some comparative data is supplied for other macruran decapods. As the form of the receptors depends to some degree upon the structure of the joint we have included details of musculature, planes of movement and degrees of freedom at each of the joints. The third maxilliped has a smaller number of chordotonal organs than the pereiopod, in particular at the mero-carpopodite and carpopodite-propodite joints where only one organ is present. In some species the propodite-dactylopodite organ is absent from this limb. The electrical activity recordable from the receptors in the 3rd maxilliped shows considerable differences from the corresponding receptors in the pereiopod. The structure of the carpopodite-propodite joint of both limbs is discussed in detail as this joint differs greatly from that of the Brachyura. In the 3rd maxilliped and 2nd pereiopod three muscles are present. In the latter the joint is capable of rotation about the longitudinal axis but the third muscle does not appear to produce this rotation. A small number of units in the CP2 receptor respond to rotation. A receptor is described in the basipodite of the pereiopod and 3rd maxilliped situated just proximal to the plane through which the limb breaks at autotomy or autospasy. This receptor does not monitor joint movement and may detect cuticular strain, thus preventing accidental autotomy of limbs. A similar receptor has been observed in Carcinus. Cuticular receptor structures (CAP organs) are described as present at the M-C and C-P joints in both limbs, and at the I-M joint of the pereiopod.