GENUS ISOTRICHA. 497 



The oral apparatus, though described as consisting of two closely approximated 

 rows of rod-like teeth, is, as distinctly indicated in Ehrenberg's original drawings, a 

 mere ovate-shaped modification only of the subcylindrical rod-fascicle distinctive of 

 the genera Nassula and Prorodon. 



Cyrtostomum leucas, Ehr. sp. PL. XXVI. FIG. 37. 



Body ovoid or ellipsoidal, usually wider anteriorly, about twice as long 

 as broad ; cortical layer enclosing numerous trichocysts ; oral aperture 

 elongate-ovate, its lower extremity pointed. Length of body 1-144". 



HAB. Fresh water. 



This species, accepted by Stein as the representative of the new genus Cyrto- 

 stomutn, is identified by that authority with both the Bursaria leucas and Bursaria 

 vernalis of Ehrenberg (Panophrys do. do. Dujardin), the latter form, as also attested 

 to by Claparede and Lachmann, being a mere variety coloured green through 

 the ingestion of chlorophyllaceous food-substances. An account of the minute 

 histology and behaviour of the trichocysts so abundantly developed in this species, 

 as carefully worked out and described by Professor Allman, is contained in the 

 section treating of these structures generally in pages 80-84 of the preceding 

 volume, while a reproduction of that authority's figures of these elements is 

 included in the supplementary plate at the end of the atlas devoted to the delinea- 

 tion of histological details. 



. 

 . GENUS IV. ISOTRICHA, Stein. 



Animalcules free-swimming, obovate, persistent in shape, depressed or 

 subcylindrical; oral aperture ventral, subterminal, followed by a short, 

 smooth, tubular, membranous pharyngeal passage ; anal aperture postero- 

 terminal ; cuticular surface clothed throughout with long, fine, densely 

 disposed cilia. Occurring mostly as endoparasites within the first stomach 

 or rumen of various higher Ruminantia. 



In general contour and habits, and in their long, dense ciliary clothing, the 

 animalcules of this genus are reported as closely resembling the Opalinidse; 

 the presence of a well-defined oral aperture serves, however, at once to distinguish 

 them from the members of the last-named group. 



Isotricha intestinalis, Stein. 



Body obovate, slightly flattened, longitudinally striate ; oral aperture 

 ventral, situated within a semilunar depression at some little distance from 

 the anterior border ; cilia long and fine ; contractile vesicles numerous, 

 distributed chiefly in the anterior region ; endoplast elongate oval, asso- 

 ciated with a small, subglobose endoplastule. Dimensions unrecorded. 



HAB. Endoparasitic, within the first stomach or rumen of sheep and 

 cattle. 



Isotricha prostoma, Stein. 



Having the oral aperture at the anterior extremity, nearly in the axial 

 line. All other details, including habitat, corresponding with thoss. 

 /. intestinalis. 



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