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The proximal leg muscles of decapod crustaceans, controlling movements at the first two joints, are anatomically more complex than the better-studied distal leg muscles. Despite extensive research on their involvement in diverse behaviors, no complete descriptions of the anatomy and innervation of these muscles for any species have been published. We describe the anatomy and innervation of the depressor muscle in the second leg of the squat lobster Munida quadrispina and compare its anatomy with that of its homologue in the crayfish Procambarus clarkii and its antagonist, the levator, in both species. Of the six anatomically distinct heads comprising M. quadrispina's depressor muscle, one arises in the cora (coxal head) and five are bi-articular (cross two joints), arising from widely dispersed sites on the thoracic endophragmal skeleton (dorsal, sternal, caudal, ventral-rostral, ventral-caudal heads). The heads' widely divergent force vectors are accommodated by the depressor apodeme's bifurcation at a thin flexible point. In total, eighteen neurons with central somata were backfilled from nerve branches to the heads. The common inhibitor and at least one neuron of unknown function with rostro-lateral soma and extremely sparse neurites innervate all heads. The sixteen excitatory motoneurons' somata are clustered in two locations, five rostral and eleven caudal to the neuropil. Rostral motoneurons innervate the two ventral heads (rostral and caudal). Their integrating segments lie rostral to those of the caudal group motoneurons and are straight or 'Y'-shaped, the latter longer and larger in diameter. Both morphological types have one prominent medial neurite that crosses the midline and could allow direct interaction between bilateral pairs of rostral motoneurons. The caudal motoneurons provide partially shared innervation to the remaining four heads. Six provide exclusive innervation, one to the caudal head, two to the sternal head, and three to the bi-articular dorsal head and uni-articular coral head which share innervation. Of the remaining five caudal motoneurons, two are shared by the caudal head and the dorsal-coral pair of heads and three are shared by the caudal and sternal heads. Fl clarkii has two depressor muscles; the rostral depressor has a single head morphologically similar to the ventral rostral head in M. quadrispina; the caudal depressor muscle has four heads (dorsal, coral, caudal, sternal) that insert on the large caudal depressor apodeme. The overall organization of depressor muscle heads in Fl clarkii and M. quadrispina is similar, taking into consideration the different internal and external thoracic anatomies and the different resting stances, horizontal and tilted, respectively. As in other species, M. quadrispina and P. clarkii have two levator muscles, each with an apodeme of complex structure attributable to the levators' role in leg autotomy. The caudal levator arises in the core; it has two heads in M. quadrispina, but only one in Fl clarkii In both species the rostral levator has three heads all arising within the thorax. Divergent force vectors and partially independent innervation of different heads composing complex musculature at single joints constitute an anatomical level of organization that neural mechanisms must accommodate to produce adaptive movements. Copyright (C) 2000 S. Karger AG. Basel.