GENUS OPISTHOTRICHA. 785 



of this genus, both distinguished from the preceding by the thickened or 

 inflated contour of their bodies and conspicuous pre-oral and endoral ciliary 

 fringes ; full details of these types, as also their specific titles, await publication. 

 The Oxytricha mystacea of Stein, with its central double row of Ventral setae, 

 is apparently regarded by Sterki as referable to the present genus, but except for the 

 somewhat irregular disposition of the component setae of these rows, it agrees in all 

 essential points with the ordinary members of the genus Holosticha of Wrzesniowski, 

 to which it is here relegated. 



GENUS XIX. OPISTHOTRICHA, S.K. 



(Greek, opisthen, behind ; thrix, hair.) 



Animalcules free-swimming, soft and elastic, ovate or elliptical ; frontal, 

 ventral, and anal styles as in Oxytricha, but bearing in addition a variable 

 number of tail-like caudal setae at the posterior extremity. 



This genus is here instituted for the reception of the Oxytricha parallela of Engel- 

 mann, and all other types corresponding with that form in the possession of several 

 caudal setae in addition to the ordinary characteristics of the genus Oxytricha. In 

 this respect it exhibits the same relationship to the last-named genus as is presented 

 by Stylonychia in comparison with Histrio. Although the single species quoted can 

 alone be referred at present to this new generic group, Sterki records his acquaint- 

 anceship with several species of Oxytricha sharing the same characteristics, the 

 specific descriptions of which, however, still await publication. 



Opisthotricha parallela, Eng. sp. PL. XLIII. FIGS. 35 AND 36. 



Body elastic and contractile, elongate-elliptical, bluntly rounded at 

 each extremity, about four times as long as broad ; peristome-field mode- 

 rately broad, extending backwards to a distance of about one-quarter of the 

 entire length ; five uncinate and three setose frontal styles ; five scattered 

 central ventral setae and a continuous peripheral border of large, scarcely 

 projecting marginal setae, the four most posterior of these so greatly exceed- 

 ing the others in length as to constitute a distinct caudal series ; anal styles 

 five in number, projecting but slightly beyond the posterior border; several 

 longitudinal rows of very fine, hair-like or hispid setae developed on the 

 dorsal surface ; contractile vesicle situated close to the centre of the left- 

 hand border; endoplasts ovate, two in number. Length 1-125". 



HAB. Fresh water. 



The great development of the fine dorsal setae constitutes a conspicuous feature 

 of this species, in addition to the presence of the four terminal or caudal setae. But 

 for the agreement in number and disposition of the ventral setas with those of the 

 ordinary Oxytrichce, this animalcule exhibits many points of affinity with Plagiotricha 

 strenua. 



Opisthotricha similis, Eng. sp. 



Body elliptical, equally rounded at each extremity ; the marginal setae 

 stationed close to and projecting beyond the lateral borders, two long sup- 

 plementary caudal setae developed at the posterior extremity ; other details 

 agreeing with those of Oxytricha pellionella. Length 1-300". 



HAB. Fresh water. 



VOL. II. 



