Brain function includes complex high-level properties, like decision-making and sensory integration, that emerge fromthe connections between neurons. Small brains, like those of the nematode Caenorhabditis elegans and of rotifers, offer,a powerful platform to measure neural functional connections in detail because of the potential completeness of their characterization. The cell-resolved knowledge of functional connectivity of a whole brain would in fact allow us to develop a deep mechanistic understanding that can be then scaled up to larger brains. As a Grass fellow, I will work onthe first steps towards expanding to Brachionus plicatilis, a species of rotifers, the methods I developed for measuring whole-brain functional connectivity in C. elegans via simultaneous optogenetics and activity imaging. My goal for thissummer is to produce transgenic rotifers expressing the fluorescent calcium indicator GCaMP in all their neurons andrecord their whole-brain activity, setting the stage for future measurements of whole-brain functional connectivity in rotifers. These capabilities will enable us to study how much functional neural circuits are evolutionarily conserved in small brains, for example the circuits responsible for sensory integration, and will be fundamental in effectively scaling up theories and models developed in small brains to the larger brains of mammals.