Vol. XLIX October, 1925 No. 4 



BIOLOGICAL BULLETIN 



TRICHODINA STEINII (C. AND L.) FROM 

 PLAN ARIA POLY CHORA (O. SCHM.). 



WM. A. KEPNER AND A. L. PICKENS, 

 UNIVERSITY OF VIRGINIA. 



In the spring of 1924 the junior author secured specimens of 

 Planaria polychora O. Schm. from a pool several hundred yards 

 west of the Biological Laboratory of the University of Virginia. 

 The Planaria, in some cases, carried upon their surfaces speci- 

 mens of Trichodina Steinii. These specimens afforded us an 

 excellent opportunity to study the habits and anatomy of this 

 species of large ectozoic peritrichous ciliate. Our observations 

 led us to recognize two interesting facts concerning this species 

 first described by Claparede and Lachmann (58). 



The genus Trichodina is so closely related to Vorticella that it 

 might be placed in the subfamily Vorticellidae. Indeed Fulton 

 (23) said of the subfamily Urceolariidae, to which Minchin 

 assigned Trichodina, "It is a matter of personal preference 

 whether one considers the Urceolariidae a subfamily of the 

 Vorticellidae ... or as a separate family." With this close 

 systematic relationship in mind, certain homologies between the 

 anatomical details of Trichodina and those of Vorticella are of 

 interest. The peristome and nucleus complex are clearly the 

 homologues of the peristome and nucleus complex of Vorticella. 

 In the free swimming Trichodina Steinii, the peristome remains 

 closed (Fig. 3) in a manner that causes it to greatly resemble the 

 closed peristome of a contracted Vorticella. A free swimming 

 Vorticella has a posterior zone of cilia (Fig. 4). The homologue 

 of these adoral cilia become fused at their bases to form the 

 "velum" of Trichodina. Trichodina Steinii, in correlation with 

 its habit as an ectozoon, has developed a horny flexible ring 

 17 237 



