GENUS PARAMECIUM. 483 



claim for admission within the ranks of the Holotricha, but to be referable, if any- 

 where among the Ciliata, to the Peritricha. Stein's second family, of the Trachelina, 

 is the only one which may be said to embrace generic groups that fairly harmonize 

 with one another ; even here, however, one out of the five genera named, Loxodes, 

 is now shown by Wrzesniowski to belong to the Hypotrichous Ciliate group, while 

 Bileptus cannot be generically separated from Amphileptus. While receiving its 

 title and definition from Professor Stein, it is worthy of remark that, in common 

 with the Peritricha, the order of the Holotricha is one of the important groups that 

 yet awaits systematic description by that eminent authority. 



The accompanying table of classification may now be submitted as embodying 

 what, in accordance with our existing knowledge of the Holotricha, would appear to 

 constitute the most natural family grouping of the numerous genera, while it at the 

 same time provides a concise clue to their distinction. In accordance with Stein's 

 classificatory system, the mouthless Opalinida; are here admitted among the Holo- 

 tricha, being regarded by the author as either peculiarly modified members of the 

 order, which by long maintenance of an endoparasitic mode of existence, have become 

 accustomed to absorb the nutrient juices of their host through the general surface of 

 their integument, and thus gradually dispensed with the need of a distinct oral 

 aperture, or which possibly, through having been primarily brought into existence 

 amid similar surroundings, have failed to develop the requirement of such inceptive 

 orifice. These alternative interpretations are discussed at length in connection with 

 the systematic description of this singular organic group. 



Fam. I. PARAM^CIID^, S. K. 



Animalcules free-swimming, more or less flattened and asymmetrical, 

 ciliate throughout, oral and cuticular cilia alike ; dorsal and ventral surfaces 

 distinct, the oral aperture opening on the ventral surface. 



Genus I. PARAMyECIUM, Miiller. 



Animalcules free-swimming, ovate or elongate, asymmetrical, more or 

 less flexible but persistent in shape, finely ciliate throughout, the cilia of 

 the oral region not differing in size or character from those of the general 

 surface of the body ; an oblique groove or buccal fossa developed on the 

 ventral surface, at the posterior extremity of which the oral aperture is 

 situated ; cortical layer usually enclosing trichocysts ; contractile vesicle and 

 endoplast conspicuous, the former under certain conditions sometimes 

 assuming a stellate outline. Inhabiting fresh and salt water, and abundant 

 in vegetable infusions. 



Paramsecium aurelia, Miill. Pl. XXVI. Figs. 28-30. 

 Body elongate-clavate or subfusiform, slightly compressed, about four 

 times as long as broad, narrower and truncate anteriorly, the posterior half 

 somewhat inflated, terminating in a conical point ; buccal groove prolonged, 

 extending from the left side of the anterior extremity obliquely backwards 

 on the ventral surface, towards the right-hand side, beyond the centre of the 

 body ; oral aperture situated at the posterior termination of the buccal 

 groove ; anal aperture ventral, situated midway between the oral orifice and 

 the posterior extremity ; contractile vesicles two in number, stellate when 

 compressed, situated respectively at a distance of about one-third of the 



