FAMILY VORTICELLID&. 655 



being more or less convolute around an elevated and protrusible central area 

 which constitutes the so-called ciliary disc ; vestibulum continued backwards 

 upon the substance of the body as a conspicuous or cleft-like pharynx, and 

 often further prolonged as a narrow, more or less distinct, tubular oesophagus ; 

 a long, solitary, setose appendage, the vestibular seta or "sole deLachmann" 

 often attached to the upper wall and projecting a considerable distance 

 beyond the entrance of the vestibulum, the entire anterior extremity of the 

 body, including the protrusible ciliary disc and oral entrance, encircled by a 

 usually conspicuously raised border or peristome, this border or peristome 

 closing sphincter-wise over the ciliary disc and oral entrance when the 

 former is retracted ; anal aperture opening into the vestibulum ; contractile 

 vesicle single, spherical, located close to the anal aperture, and debouching 

 upon the vestibulum ; endoplast mostly band-like ; increasing usually by 

 longitudinal, rarely by transverse fission, the liberated zooid in these 

 instances usually developing prior to, and retaining during its short free- 

 swimming existence, a posterior circlet of powerful locomotive cilia, which 

 are again absorbed on a suitable site for attachment being secured ; the 

 survival or rejuvenation of the species further provided for through the conju- 

 gation or genetic union of two dissimilar zooids, the one (male ?) minute and 

 migrant, and the other (female ?) normal and sedentary, and by develop- 

 ment out of the endoplast of minute, free-swimming germs. Inhabiting 

 salt and fresh water ; rarely, if ever, possessing trichocysts. 



The family of the Vorticellidae represents one of the largest and at the same time 

 the most natural and typical group of the Peritrichous order. All its members are 

 at once recognized by their normal sedentary condition and by the characteristic 

 structure of the oral system detailed at length in the foregoing diagnosis and in 

 that of the more essentially representative genus Vorticella. In but few of the 

 numerous genera included in this family is there any marked divergence from this 

 formula, that which exists being manifested by a greater development of some one 

 element at the expense of another constituent part of this last-named system. 

 Thus, in the genus Spirochona, the external edge of the prominent encircling border 

 or peristome is mostly suppressed, while its inner one is abnormally developed as a 

 transparent and highly elevated spiral membrane. On a less conspicuous scale a 

 similar deviation of the peristome-border from its more normal form obtains in the 

 genera Pyxidium, Opereularia, and Lagenophrys ; the ciliary disc, however, exhibits 

 an additional modification in these three instances, being isolated, laterally attached, 

 and presenting in its extended condition the aspect of a stalked operculum. By 

 Ehrenberg, Siebold, Perty, and other early writers the family of the Vorticellidae as 

 here constituted is separated into two or even three groups of equivalent value; 

 Vaginicola and its loricate allies being comprised under the family title of either the 

 Ophrydina or Vaginifera, the first of these two titles being reserved by Perty for the 

 reception only of Ophrydium, By Stein even,* a similar separation of the naked 

 from the loricate or mucus-immersed forms is maintained, though that the two are 

 too intimately related for such a broad separation is amply exemplified by the 

 following examples. Thus in certain species of Opereularia, such as O. nutans and 

 O. berberina, the cuticular surface is considerably indurated, and remains, like the 

 empty sheath of a Cothurnia, after the decay of the enclosed parenchyma ; while in 

 Lagenophrys the adnate lorica is scarcely distinguishable from an indurated cuticle. 

 Through Gerda fixa, and the newly described Ophionella picta, on the other hand, 



* ' Organismus,' Abth. ii., 1867. 



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