1. The structure of the myelinated fibres of prawns is described, and the homologies of the nucleated sheath which lies between the axon and the fatty layer discussed.

2. The relative thickness of the myelin sheath increases with decrease in total diameter of the fibre along a curve similar in shape to that found in vertebrates and earthworms.

3. Nodes of Ranvier are found in the sheaths of most fibres of a diameter greater than about 13µ

4. The nodes are similar to those in vertebrate nerves in that the myelin sheath is interrupted at the node.

5. The conduction velocity of fibres in the central nervous system of axon diameter 26µ and total diameter 35µ is between 18 and 23 m. per sec., a rate faster than is found in the "unmyelinated" fibres of similar size in other crustacea.

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