Lipid Composition of the Nervous System of Earthworms (Lumbricus terrestris)

As part of a systematic study of the evolution of the nervous system, the lipid composition of the ventral nerves of earthworms was examined. The nerve axons are wrapped in copious layers of loosely bound membrane, superficially resembling the myelin sheath of vertebrates. However, neither galactocerebroside nor sulfatide, both of which are considered to be markers for myelin, was present, and only traces of glucocerebroside, which is abundant in shrimp nerve, were detected. The lipids were rich in cholesterol (15.3 mumol/g of fresh tissue) and phospholipids (21.7 mumol/g of fresh tissue). The phospholipids were composed of phosphatidylethanolamine, -choline, -serine, and -inositol in the ratio of 41:44:8:7. Most of the ethanolamine-containing phospholipids were in the form of plasmalogens. The fatty acid moieties of these phospholipids were predominantly 18:1, 18:0, and 20:1, whereas the aldehyde moieties of plasmalogen were mostly 18:0. Sphingomyelin, which is considered a ubiquitous component of animal membranes, was not detected. How the unique structure of the membranes of earthworm nerves may be related to the function of the nervous system in this organism is discussed.