Since the publication of Claparède’s classical memoir on the earthworm, numerous writers have contributed to advance our knowledge of the morphology of this animal. In England, the writings of Ray Lankester, W. B. Benham, and F. E. Beddard have considerably increased the literature of the subject; but, with the exception of incidental references to function, the papers hitherto published have dealt chiefly with the morphology, taxonomy, or geographical distribution of the animal. Darwin long ago collected the observations of Claparède and other older authorities on the physiology of the earthworm, in his work on Vegetable Mould, which consists mainly of his own observations on the habits of the animal. An article “On the Retractile Cilia in the Intestine of Lumbricus terrestris ,” by M. Greenwood, in the ‘Journal of Physiology,’ vol. 13, is, I think, the only systematic contribution in English to our knowledge of the physiology of the earthworm in recent years. The present communication contains the results of an investigation into the functions of the cœlomic fluid. The greater part of the research consists, therefore, of a detailed study of the morphology and physiology of the amœboid cells, which form the most important contents of the cœlom. But in the course of my observations, I was led to investigate some of the chemical characters of the cœlomic fluid, and also to examine certain other structures, such as the skin and the dorsal pores, which form, with the cœlomic fluid, a very remarkable protective mechanism.