ON PERICHAETIDAE. 201 



NOTE XXIX. 



ON THE VARIABILITY OF CHARACTERS IN 

 PERICHAETIDAE 



BY 



Dr. R. HORST. 



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 diSerences 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 ^) as well 

 as Benham ^) 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 varia- 

 bility of some characters always used in differentiating 

 Perichaeta-apecies , viz. the number of spermathecae, and 

 the number and arrangement of copulatory papillae. 



1) Oligochaeten in Kükenthal's Ergebn. einer Zodog. Forschungsreise in den 

 Molakken und Borneo; Abhandl. Senckenb. naturf. Gesellsch. Bd. XXllI, 

 1896, p. 308. 



2) Journ. Linnean Society, Zoology, vol. XXVI, p. 221. 



Notes from th.e Ley den IMuseuui, Vol. XX. 



13** 



