Open Access

John I. Hamaker

• 1. The central nervous system of Nereis virens occupies a deeper position. than does that of most Polychaetes. It is separated from the hypodermis by the circular muscles, and is enveloped by an elaborate protective tissue. 2. The protective tissue consists of two parts jan inner spongy layer, the neuroglia, of ectodermic origin, and an outer sheath, the neurilemma, of mesodermic origin. 3. The" mushroom bodies" of insects and decapod Crustacea are represented in the brain of Nereis by the anterior masses of small nuclei. 4. The optic ganglion, which in some species of Nereis lies beneath the anterior eye, may in other species lie within the brain capsule. 5. There is no neuropil in the ventral nerve cord. 6. There are three longitudinal connectives between each two successive ganglia of the ventral nerve cord, one small median and two larger lateral ones. 7. The sheaths of the nerve fibres of the ventral cord have no nuclei, and hence must be a product of the fibres themselves. 8. The nerve cells of the ventral cord commonly have one or more centrosomes. 9. The giant fibres are nervous in function, and are put into relation with peripheral organs through ordinary centrifugal fibres. 10. The giant fibres give off no fibrillations, and nervous relation with other fibres is established directly between the axis cylinders. 11. Certain decussating fibres are always united in pairs by anastomoses between the axis cylinders where they cross each other. 12. Certain centripetal fibres of the same set are always united by anastomoses between the ends of the branches. 13. Contact between axis cylinders may possibly be one of the means of bringing nerve fibres into functionall'elation with each other.