_f.br.r._, terminal fibre-layer of retina; _n.l.r.g._, bipolar cells of the ganglion of the retina = inner nuclear layer; _m.l._, Punktsubstanz = inner molecular layer; _b.m._, basement membrane formed by neurilemma round central nervous system.] Thus, according to Parker, the mass of nervous tissue which occupies the central part of the optic stalk in Astacus is composed {92}of four distinct ganglia; the retina is connected with the first of these by means of the retinal fibres, and the optic nerve extends proximally from the fourth ganglion to the brain. Each ganglion consists of ganglion-cells, nerve-fibres, and 'neuropil,' and, in addition, supporting cells of a neuroglial type. By means of the methylene blue method and the Golgi method, it is seen that the retinal end-cells, with their visual rods, are connected with the fibres of the optic nerve by means of a system of neurones, the synapses of which take place in and help to form the 'neuropil' of the various ganglia. Thus, an impulse in passing from the retina to the brain would ordinarily travel over five neurones, beginning with one of the first order and ending with one of the fifth. He makes five neurones although there are only four ganglia, because he reckons the retinal cell with its elongated fibre as a neurone of the first order, such fibre terminating in dendritic processes which form synapses in the 'neuropil' of the first ganglion with the neurones of the second order. Similarly the neurones of the second order terminate in the 'neuropil' of the second ganglion, and so on, until we reach the neurones of the fifth order, which terminate on the one hand in the 'neuropil' of the fourth ganglion, and on the other pass to the optic lobes of the brain by their long neuraxons--the fibres of the optic nerve. He compares this arrangement with that of Branchipus, Apus, Estheria, Daphnia, etc., and shows that in the more primitive crustaceans the peripheral optic apparatus was composed, not of four but of two optic ganglia, not, therefore, of five but of three neurones, viz.-- 1. The neurone of the first order--_i.e._ the retinal cell with its fibre terminating in the 'neuropil' of the first optic ganglion (ganglion of the retina). 2. The neurone of the second order, which terminates in the 'neuropil' of the second ganglion (ganglion of the optic nerve). 3. The neurone of the third order, which terminates in the optic lobes of the brain by means of its neuraxons (the optic nerve). We see, then, that the most recent researches agree with the older ones of Berger, Claus, and Bellonci, in picturing the retina of the primitive crustacean forms as formed of two ganglia only, and not of four, as in the specialized crustacean group the Malacostraca.