The stomatogastric nervous system of the reptantian Decapoda Crustacea, particularly the small isolated stomatogastric ganglion containing the 25-30 motor neurons that control the muscles of the gastric mill and the pyloric filter of the stomach, is an important preparation for research in comparative neurophysiology. Unfortunately there are no comprehensive descriptions of the neuromuscular system of the stomach in these animals. Therefore, since the stomatogastric motor neurons are identified by reference to the muscles they innervate, it has been difficult to identify neurons within or between species. The most important features for classifying the muscles of the decapod stomach are the ossicles to which the muscles attach. In the latter part of the last century Mocquard demonstrated that the stomach ossicles of the decapods could be compared in different groups despite the large variations from group to group. A summary of Mocquard's (1883) classification scheme, with some modifications, is given. The scheme recognizes 33 ossicles in seven categories (cardiac gastric mill, I-VII; lateral supporting cardiac ossicles, VIII-XV; ossicles of the cardio-pyloric valve, XVI-XVIII; supporting ossicles of the dorsal pyloric stomach, XIX-XXI; supporting ossicles of the ventral pylorus and ampullae, XXII-XXVII; supra-ampullary ossicles, XXVIII-XXX; supporting ossicles of the lateral pylorus, XXXI-XXXIII). Where necessary, comments are then made on the ossicles of the three divergent species studied, the blue crab, Callinectes sapidus (Brachyura); the lobster, Homarus americanus (Macrura) and the spiny lobster Panulirus argus (Palinura). Most of the thirty-three ossicles are found in each of the species, but there are some major differences between species. Callinectes, for example, has the most complex ossicle system and Panulirus the most reduced. A complete description of the stomach muscles of the three species is then presented. A new and flexible terminology is used in these descriptions. The stomach muscles are divided basically into five major groups by their location on the gut (cardiac sac, gastric mill, ventral cardiac, cardio-pyloric valve and pyloric). Callinectes has the most complex muscle system with 41 muscles and 51 muscle bundles, Homarus has 37 muscles and 49 muscle bundles, and Panulirus has 38 muscles but only 44 muscle bundles. Nevertheless it was clearly possible to identify homologous muscles in these species, at least thirty muscles being common to all three species. Most of the variation between species occurs in the intrinsic muscles (those confined to the stomach wall) of the gastric mill and the cardio-pyloric valve groups. The gross anatomy of the stomatogastric nervous system is described for the three species. In the main the earlier terminology of Orlov was used but it was also convenient to classify the system into four broad groups (class 1, oesophageal motor neurons; class 2, sensory fibres; class 3, neurosecretory fibres; class 4, the stomatogastric ganglion and its associated input and output nerves). Again it was not difficult to identify the same gross nerve trunks in the three species. This was particularly true of the major axis of the stomatogastric ganglion system (stomatogastric nerve, stomatogastric ganglion, dorsal and lateral ventricular nerves, pyloric dilator nerve, and the pyloric nerve) of class 4 and of the general course of the postero-lateral nerve and posterior stomach nerve of class 2. Differences between the three species do occur, for example in the branching pattern in the pyloric region and the nature of the lateral nerves leaving the stomatogastric ganglion and dorsal ventricular nerve. In Panulirus an almost complete description of the innervation of the stomach muscles by the individual stomatogastric ganglion motor neurons is also given. This description is partly based on physiological data published elsewhere. Where possible the description in terms of the motor neurons is also given for Homarus and Callinectes, but present physiological data are incomplete for these species. It should be noted that there is evidence that some homologous stomatogastric ganglion motor neurons travel along quite different nerve trunks in the different species. In Homarus, for example, the median ventricular nerve carries the axons of the ventricular dilator and lateral cardiac motor neurons, whereas in Panulirus this nerve carries the axons of the ventricular dilator and the inferior cardiac motor neurons. In conclusion summary tables and diagrams are presented for the muscle and nervous anatomy, and it is suggested that the classification schemes presented here should provide the basis for consistent identification of the stomatogastric motor neurons throughout the reptantian decapods.