Sea slugs are a well characterized model system for studying the functions of the nervous system. It's nervous system is relatively simple, compared to humans, but the connections between nerves and the mechanisms by which neurons communicate with each other is similar. Also, the neural mechanisms used in learning and memory can be studied in this organism.
Up until now, the study of the sea slug's nervous system has been limited due to a lack of genetic and genomic information. No more. Researchers from Columbia University and the University of Florida have characterized up to 70% of the sea slug's neuronal transcriptome by sequencing neuronal cDNA libraries. This provides a new database resource for studying the functions of sea slug's nervous system, including the processes involved in behavior, learning and memory, but also allows comparative analyses with other species, including humans. As a first indicator of the importance of this new resource, many of the genes that have been implicated in important human maladies such as Parkinson Disease and Alzheimer Disease have been shown to be conserved and expressed within the sea slug's nervous system.
You can get more information on this important work. The full report was published in the Dec 29, 2006 issue of Cell. The PubMed abstract is available online. An accessible summary of this work is available via LiveScience.