The nerve cells of theAplysiaare much larger than mammalian neurons. Using theAplysiaganglia tostudy the relationship between the cellular structure and the diffusion MRI signal can potentially shedlight on this relationship for more complex organisms. We measured the dMRI signal of chemically-fixed abdominal ganglia of theAplysiaat several diffusion times. At the diffusion times measured andobserved at low b-values, the dMRI signal is mono-exponential and can be accurately represented bythe parameter ADC(Apparent Diffusion Coefficient). We performed numerical simulations of waterdiffusion for the large cell neurons in the abdominal ganglia after creating geometrical configurationsby segmenting high resolution T2-weighted(T2w)images to obtain the cell outline and thenincorporating a manually generated nucleus. The results of the numerical simulations validate theclaim that water diffusion in the large cell neurons is in the short diffusion time regime at ourexperimental diffusion times. Then, using the analytical short time approximation(STA)formula forthe ADC, we showed that in order to explain the experimentally observed behavior, it is necessary toconsider the nucleus and the cytoplasm as two separate diffusion compartments. By using a twocompartment STA model, we were able to illustrate the effect of the highly irregular shape of the cell nucleus on the ADC.