Quantitative investigations on variability of visual morphological parameters in neurons BB1, BB3-5 and clusters BBb have been performed on the ventral side of the visceral ganglion in 20 molluscs Lymnaea stagnalis L. (height of the shell -45.5-1.3 mm, variation coefficient -2.9%) cought in the same pond and on the same day. Statistical analysis of the data obtained is performed by means of the methods applied for analysing small independent samples. A probability character of visual identification and an essential variability are demonstrated for the parameters of the neuron BB1 (diameter, position, colour), of the number of giant and/or large neurons BB3-5 and the cluster BBb neurons. The variation character of the parameters mentioned is asymmetric. It is possible to speak on a greater determinancy of a preferable gradation of the parameters. The interconnection of the variability and the determinancy of the morphological signs with the identificity and functioning of the neuronal elements are discussed. A suggestion is made that identification of the neurons according to both their morphological and functional criteria, as a whole, can have a probability character. The data considered in the work make doubt the invariance of the neurons studied in the mollusc Lymnaea stagnalis L.