Serotonin (5-hydroxytryptamine, 5-HT) has long been associated with the vertebrate gut and is an important neuromodulator of crustacean foregut. This dissertation presents evidence that 5-HT initiated peristalsis in crayfish hindgut and enhanced the power of contractions in caudal regions of the hindgut. 5-HT receptor immunoreactivity studies showed that the two identified crustacean 5-HT receptors, 5-HT1α and 5-HT2β are present on the hindgut in different and distinctive patterns. 5-HT immunoreactivity (5-HT-ir) studies revealed that the fibers from central neurons found on the hindgut showed a broad range of 5-HT-ir intensity, which led to the hypothesis that they borrowed 5-HT. This hypothesis was tested by first determining that the HGNs can take up 5-HT through a serotonin transporter and that uptake can be blocked by a serotonin reuptake inhibitor. Second, synthesis was tested by superfusing tryptophan and using 5-HT-ir to determine the presence of 5-HT. No constitutive 5-HT synthesis occurred under these conditions. Superfusion of the intermediate product of 5-HT synthesis, 5-hydroxytryptophan (5-HTP), did lead to 5-HT-ir. The HGNs can take up 5-HT but have only one of the synthetic enzymes. The lack of nearby sources for 5-HT led to the hypothesis that hormonally supplied 5-HT may be the source for 5-HT in the HGNs. High performance liquid chromatography measurements of 5-HT and 5-HTP levels in tissue following injection of 5-HT into the hemolymph revealed that levels of 5-HT significantly increased in the terminal ganglion and hindgut, where the HGNs cell bodies and projections are respectively located. All other areas of the central nervous system, with the exception of the brain, also showed a significant increase in 5-HT levels. Injection of tryptophan produced a significant increase in 5-HTP levels in the brain. Quantitative 5-HT-ir indicated that feeding increased the intensity of 5-HT-ir in the HGNs. Feeding was determined to be a relevant stimulus to examine facultative synthesis of 5-HT. The enzyme that converts 5-HT to 5-HTP was blocked and 48 hrs after feeding 5-HTP-ir was used to indicate that facultative synthesis did not occur. At the same time, 5-HT-ir was used to indicate that uptake of 5-HT by the HGNs more likely occurred.