Structure of the Brain and Medulla Terminalis of the Spiny Lobster Panulirus Argus and the Crayfish Procambarus Clarkii, With an Emphasis on Olfactory Centers

We present a comparative morphological study of the brain and medulla terminalis (fourth eyestalk ganglion) of the spiny lobster Panulirus argus and the crayfish Procambarus clarkii as a basis for investigations of the functional organization of their olfactory systems. Both the brain and medulla terminalis of P. argus display folding with respect to those of P. clarkii, which alters the relative locations of analogous soma clusters, tracts, bundles, and neuropiles. The major olfactory neuropiles of the central nervous system are the olfactory lobes and the accessory lobes in the brain, and the hemiellipsoid bodies and regions IV of the glomeruli centrales in the medullae terminales. All of these paired regions of neuropile have a glomerular organization. The glomeruli of the olfactory lobe are radially arranged, and each glomerulus is divided into distinguishable regions. The glomeruli of the accessory lobe are smaller and more numerous than those of the olfactory lobe, are spherically shaped, and are organized into three groups within each accessory lobe (this tripartite arrangement is much more obvious in P. argus than in P. clarkii). The medulla terminalis contains 13 distinct neuropile regions, including the hemiellipsoid body and region IV of the glomeruli centrales, both of which are interconnected with the olfactory lobes and accessory lobes via the olfactory-globular tract and contain numerous microglomeruli.