·Minireview·Memory, one of the important properties of central nerve system (CNS), is defined as encoding, storage and retrieval of learned inputs. However, many complex processes of memory still need further scientific explanations. Invertebrate animals have been already widely accepted as successful organisms for the study of neuroscience, and many important discoveries on behavioral cognitive science were made in the model invertebrate animals [1,2] . Model invertebrate organisms such as Caenorhabditis elegans and Drosophila melanogaster have provided powerful genetic approaches to a series of central questions concerning neural development, learning and memory, and the cellular and molecular substrates of behaviors [3][4] . In this review, we will discuss the recent progress on the molecular control of memory based on the studies of model organism C. elegans, via different paradigms in which diverse environmental clues are involved. C. elegans has become an ideal organism model to unravel the complex processes of learning and memory [5] . Besides its extensively anatomical knowledge, the neural circuit of the worm can be analyzed by the technique of laser ablation for identified neurons. Its entire genome has been mapped and sequenced, which allow us to use the methods of systematic forward or reverse genetic screen to elucidate the molecular mechanisms of memory. Especially, C. elegans displays simple forms of memory, such as thermotaxis, chemotaxis and mechanotransdution [5] .
Memory for thermosensationC. elegans depends on environmental temperature to sense and search food. Therefore thermotaxis may be the most powerful behavioral paradigm for elucidating the memory mechanism underlying this behavioral plasticity. That model gives an association between the temperature encoded into memory and the nutritional state, and the temperature-food association can be kept for a long period [6] . This thermosensory system exhibits properties of exquisite temperature sensitivity [storage of the thermotactic set-point (T s )], longterm plasticity (comparison of current temperature with the T s ), and the ability to transform thermosensory input into different patterns of motor neurons (measurement of thermal gradients during movement). In brief, the animals adjust a stored set-point of thermotactic memory to their cultivation temperature (T cult ) and the AFD (a sensory neuron)-AIY (an interneuron, the major postsynaptic partner of AFD)-AIZ (an interneuron, the major postsynaptic partner of AIY) neuron circuit: the AFD and AIY neurons specify the thermophilic components of the circuit, driving movement to higher temperatures in a thermal gradient; the AIY and AIZ neurons specify the cryophilic components of the circuit, driving movement to lower temperatures in a thermal gradient [7] . The time-course assay of thermotaxis suggested that a new temperature encoding is a relative long process, and which appears to be independent of cultivation temperature [6] . In addition, starvation can accelerate the memory formation...