The goal of my thesis is to further understanding of the highly sophisticated yet relatively uncharacterized visual system of the octopus. To examine the functional organization of this system, I used electrophysiological recordings to document the response of groups of photoreceptors in the retina to polarized light stimulation. To examine in more detail the organization of the neurocircuitry that underlies visual processing in higher levels of the octopus visual system, I documented the expression patterns of a host of transcripts of key enzymes and receptors in the glutamatergic, cholinergic, and to a lesser degree in the dopaminergic, GABAergic, and octopaminergic neurotransmission pathways within the optic lobe. ADAPTATION AND FACILITATION IN THE RESPONSE TO POLARIZED LIGHT IN THE VISUAL SYSTEM OF OCTOPUS BIMACULOIDES. In an oceanic world of diminished color and contrast, polarized light detection is a powerful tool for overcoming visual challenges. Studies are making it increasingly clear that many animals use polarization information from light, for purposes ranging from detecting migration cues, to detecting prey and predators, to interspecies communication. Behavioral studies have documented the ability of cephalopods to not only detect differences in polarization of light, but in addition to alter the polarization of light incident on their iridophores, hinting at the possibility of a concealed communication that could exist with such intentional patterning. Electrophysiological recordings have indicated that polarization angle is indeed represented in the output signal of individual rhabdomeric photoreceptors within the cephalopod retina. However, the connectivity of those photoreceptors, or how they may process polarization information on the level of the retina, remains poorly understood. I tested both the adaptation and facilitation effects of stimulation with polarized light on groups of photoreceptors. I report that facilitation, consistent with a residual calcium hypothesis, occurs in short-term responses to stimulation. Over longer time courses of stimulation, adaptation effects on photoreceptor output occur independently of the polarization orientation of the adapting light, indicating that photoreceptors are connected to and communicating with each other in the retina to a much greater degree than previously postulated. The results I report here are the first indication that even on the level of retinal output, octopuses potentially possess not only polarization sensitivity, but also the capacity for true polarization vision. CHARACTERIZATION OF COMPONENTS OF THE GLUTAMATERGIC SIGNALING PATHWAY IN THE OPTIC LOBE OF OCTOPUS BIMACULOIDES: Glutamate is a simple molecule, which over the course of evolution has been recruited widely across the animal kingdom for use as a neurotransmitter. A diversity of expression of glutamate receptors allows for a wide variety of effects of glutamate on various cell populations, and underlies much of the subtle sorting of information that takes place in our own retinas. NMDA-type glutamate receptors have evolved to play a key role in memory and learning circuits. Glutamate has been long known to be present in the cephalopod visual system, but its specific roles and paths within the neurocircuitry therein remain poorly understood. Here I identify and document the expression pattern of the synthetic enzyme of glutamate, glutamine synthetase, as well as a host of receptors subunits from AMPA, kainate, and NMDA-type glutamate receptor subunit families. These data allow for more precise identification of glutamatergic cells in this system than was previously possible. CHARACTERIZATION OF COMPONENTS OF THE CHOLINERGIC SIGNALING PATHWAY IN THE OPTIC LOBE OF OCTOPUS BIMACULOIDES: Acetylcholine is also used across the animal kingdom as a neurotransmitter. The evolution of fast acting nicotinic acetylcholine receptors has allowed for this molecule to play a key role in rapid signal transduction in a large variety of neural pathways. In particular, acetylcholine is present in high concentration in the cephalopod optic lobes, and is has long been thought that acetylcholine plays a critical part as a neurotransmitter in cephalopod visual processing. Here I identified and documented the expression pattern of mRNA for the synthetic and packaging enzymes necessary for presynaptic cells to produce acetylcholine, as well as a host of acetylcholine receptor subunits present in the optic lobe. Of particular interest is the documentation of members of a novel class of inhibitory acetylcholine receptor subunits that has arisen independently in the molluscs. These data further the hypothesis that acetylcholine is indeed used prevalently in this system. THE IDENTIFICATION AND EXPRESSION PATTERNS OF COMPONENTS OF THE DOPAMINERGIC, GABAERGIC, AND OCTOPAMINERGIC NEUROTRANSMISSION PATHWAYS, AND A NOTE ON INHIBITORY GLUTAMATE RECEPTORS IN THE OPTIC LOBE OF OCTOPUS BIMACULOIDES: Many other classical neurotransmitters have been implicated in cephalopod visual processing, and here I present further data elucidating the cellular populations that participate in signaling using dopamine, GABA, and octopamine. Identification of the expression pattern of mRNA for both the synthetic enzyme and the signal-degrading transporter for dopamine implicates a small population of cells in the optic lobe as dopaminergic. Similarly, expression of mRNA for the degradative enzyme of GABA highlights a small but widespread population of cells that act to remove GABA from their synapses, presumably after receiving its signal. Though named for the octopus, octopamine presence and utility is poorly understood in the central brains of these animals; here I show the first documentation of the expression of the synthetic enzyme for octopamine within the optic lobe. The additional identification of inhibitory glutamate receptor subunit sequences in the transcriptome is also discussed.