Every one occupied with the study of Perichaetidae, no doubt will be persuaded, that too much species of this genus are based on a single specimen, that showed some slight differences from well-known type-species, whether by its being not quite mature, or by the variability of some of its organs. In a recent paper Michaelsen 1) as well as Benham 2) especially called attention to this fact and urged the necessity of examining as many individuals of a species as possible, to augment our knowledge of the variability of the characters of these worms and of their value for the discrimination of the species. Whether Michaelsen is right in uniting so many species of Perichaeta, as done by him in his paper, above referred to, I hope to discuss later on; for I think Benham rightly says: »until we know more of the variability of the animals we are justified in regarding a given position as fixed, if any considerable number of specimens reveal it.” In the present paper I wish to point out for a couple of them the variability of some characters always used in differentiating Perichaeta-species, viz. the number of spermathecae, and the number and arrangement of copulatory papillae. Perichaeta biserialis Perrier.