It is to Wharton Jones that we owe the first comprehensive account of the leucocytes in the various groups of Invertebrata : in an important paper, published in 1846 (6), he described and figured them as of “stellate appearance,” and sometimes as shooting out “cilia-like processes.” Since then numberless authors have studied these cells and pictured them as provided with free outstanding pseudopodia. Evidence is brought forward in this paper that the spiny appearance of leucocytes is of the nature of an optical illusion or due to changes taking place under abnormal conditions, that freely projecting fine processes are seldom, if ever, produced in the fluids of living invertebrates, and that the pointed pseudopodia so often figured are merely the optical sections of delicate, more or less folded films.