GENUS TRICHODINA. 645 



Fam. VI. URCEOLARIID^, Stein. 



Animalcules free-swimming or adherent at will, discoidal, turbinate, or 

 hourglass-shaped ; the anterior border more or less circular, associated with 

 a spirally convolute ciliary wreath, the right limb of which is usually 

 involute, and descends into the oral aperture ; oral system closely 

 resembling that of the Vorticellidae, consisting usually of a widened 

 anterior entrance or vestibulum, and a somewhat prolonged pharyngeal 

 passage ; posterior border acetabuliform and adhesive, ciliate round its 

 peripheral margin, and mostly strengthened internally with a simple or 

 denticulate horny ring. 



With the typical genera of this family, TricJiodma and Urccolaria, are here included 

 two additional forms, Cydochcefa and Liawphora, which exhibit in connection with 

 their oral systems a marked deviation from these nomial types. In the first of 

 these, the anterior adoral ciliary wreath is entirely obliterated, or represented only by 

 that extremity which descends into the greatly prolonged pharyngeal passage. In 

 Licnophora it is the left-hand limb of the adoral wreath instead of the right, as in most 

 Peritricha, that is spirally convolute, a circumstance which, together with the oblique 

 setting and more elongate contour of the peristome-field, likens it, as pointed out by 

 Claparede, to the corresponding region in Stylouychia and other Hypotricha, The 

 adoral ciliary system in the two earlier established genera first named closely 

 resembles that of the ordinary Vorticellidae, but is more simplified through the non- 

 existence of a protrusible ciUary disc. To these last-named animalcules, in their 

 free-swimming state, having a supplementary circlet of locomotive cilia temporarily 

 developed at their posterior extremity, the Trichodina, indeed, exhibit a most remark- 

 able likeness. Such temporary free-swimming Vorticellidae, as here illustrated, more 

 especially in the case of Epistylis digitalis, see PI. XXXVIII. Fig, 16, are able in 

 a sigiilar manner to adhere to, and creep over the surface of submerged objects 

 with the aid of their temporarily flattened posterior extremity and adventitious ciliary 

 circlet, which may without doubt be regarded as homologous with the ciliary wreath 

 that permanently fringes the acetabuliform posterior extremity of Trichodina and its 

 allies. 



All the representatives of this family group are distinguished by their essentially 

 parasitic or commensal habits. 



Genus I. TRICHODINA, Ehrenberg. 



Animalcules free-swimming, highly elastic and changeable in shape, 

 conical or discoidal ; oral aperture terminal, eccentric, continued into a 

 cleft-like pharynx, and associated with a horizontal, spirally convolute, 

 adoral wreath of cilia, the right limb of which is involute and descends into 

 the oral aperture ; anal orifice debouching into the vestibulum ; posterior 

 extremity discoidal, acetabuliform, its outer margin fringed by a circle of 

 long even cilia, its inner border strengthened by a more or less indurated 

 horny ring and supplementary denticles ; endoplast band-like or monili- 

 form ; contractile vesicle spherical, located near the termination of the 

 pharynx. Inhabiting salt and fresh water, usually infesting the cuticular 

 surface of hydroid zoophytes and other Invertebrata. 



