542 ORDER HOLOTRICHA. 



figures and further details of the form and structure of the two specific types are sup- 

 plied such a separation is premature. Stein suggests that the posterior membranous 

 border in this species is possibly the analogue of the membranous sucking-disc of 

 the Trichodinopsis paradoxa of Claparede and Lachmann, which occurs under similar 

 conditions within the intestinal tract of Cydostoma elegans. On the other hand, it 

 would appear to exhibit an affinity with the genus Conchophthirus. The position 

 and structure of the so-called anal aperture, which exhibits so remarkable a modi- 

 fication in the preceding species, has not been determined in the present form. 



Fam. X. PLEURONEMID-ffi, S. K. 



Animalcules free-swimming, more or less ovate, ciliate throughout ; oral 

 cilia diverse in character to those of the cuticular surface; oral aperture 

 terminal or ventral, supplemented by an extensile and retractile hood- 

 shaped membrane or velum. 



GENUS I. PLEURONEMA, Dujardin. 



Animalcules ovate; oral aperture situated in a depressed area near 

 the centre of the ventral surface, supplemented by an extensile hood- 

 shaped transparent membrane or velum, which is let down or retracted at 

 the creature's will ; numerous longer vibratile cilia stationed at the entrance 

 of the oral cavity ; the general surface of the body clothed with long, stiff, 

 hair-like setae, occasionally accompanied by a few additional and longer 

 setae at the anterior extremity ; no abnormally developed posterior or caudal 

 seta ; the cortical layer usually containing trichocysts ; contractile vesicle 

 single, anteriorly situated. 



By but few of the investigators of the Infusorial class of the Protozoa, up to the 

 present date, has the structural organization of the genus Pleuronema and its 

 allies been correctly estimated. The type of the genus, the Paramcerium chrysalis of 

 Ehrenberg, as first separated by Dujardin and subsequently studied by Perty and 

 Claparede and Lachmann and De Fromentel, has been altogether misinterpreted. 

 All these authorities agree in characterizing this form as being distinguishable from 

 Param&riunii merely in the development around the ventrally situated oral fossa of 

 numerous, abnormally long, recurved, retractile and vibratile cilia. That the outermost 

 of these so-called recurved vibratile cilia truly mark the peripheral contour of a 

 delicate hyaline membrane, has been conclusively ascertained by the present author, 

 and it is gratifying to find that Stein, in the very brief reference he makes to the 

 genus in the second volume of his ' Organismus,' expresses a corresponding view. This 

 interpretation of the characteristics of the genus was arrived at so long since as the year 

 1870, before having access to Stein's observations, and has been reconfirmed upon 

 innumerable occasions. An identical type of structure has likewise been found by 

 the author to obtain among the representatives of the genera Cyclidium and Uronema 

 of Dujardin, and exists also apparently in the genus Baonidium of Fresenius. This 

 transparent extensile membrane is by no means identical in form or function with 

 the tremulous flap or membrane possessed by the ciliate animalcules previously 

 described. Among these, this oral membrane has mostly an independent vibra- 

 tory action, and is immediately subservient to the purpose of bringing food-particles 

 to the mouth. In the case of Pleuronema, on the other hand, such membrane 

 is not vibratory, but fulfils the purpose of an exquisitely constructed bag-like trap, 

 raised or lowered at will, into which food-particles are swept by the action of the 

 special longer cilia stationed close to the mouth and enclosed within its borders. In 



