764 ORDER HYPOTRICHA. 



amended by Stein and here abbreviated, the distinction is shown to be dear and 

 decisive. It was originally premised that this species preyed upon the Uving tissues 

 of the polype which serves it as a host, the characteristic thread-cells of the higher 

 organism being frequently found inside it. Such structures, however, are only 

 incepted with other waste matter thrown off from the surface of the polype's integu- 

 ment, and in the removal of which its tiny guest undoubtedly plays the role of a 

 useful scavenger. Other food-matters, such as monads, desmids, and diatoms, are 

 devoured with equal avidity, and form an important addendum to its customary bill 

 of fare. 



The generic title of Alastor was proposed for this type by Max Perty. 



Genus III. TRICHOGASTER, Sterki. 



Animalcules free-swimming, more or less ovate, the entire ventral 

 surface clothed with minute setose cilia, a few of slightly larger size repre- 

 senting the frontal, and four or five of similar size the anal series. 



Trichogaster pilosus, Sterki. 



Body elongate oval ; peristome as in Urostyla, extending backwards to 

 one-third of the length of the entire body ; adoral cilia short, fine, and very 

 closely set. Length i-i 1 5". Hab. — Fresh water. 



A single example only of the animalcule upon which this genus and species is 

 founded has been met with by Sterki, and as indicated by that writer,* it more 

 nearly approaches Urostyla and Kerona in the character and distribution of its ciliary 

 appendages than any other representative of the Oxytrichidae. Except, indeed, that 

 the ventral sets were not determined to be distributed in even longitudinal rows, 

 and that the frontal and anal styles are relatively smaller, there would appear, 

 in the absence of an illustration or fuller descriptive details, to be little distinction 

 between this type and the ordinary members of the genus Urostyla. 



Genus IV. UROSTYLA, Ehrenberg. 



Animalcules free-swimming, flexible and elastic, ovate or elongate, 

 rounded at the two extremities, bearing three or more uncinate frontal 

 styles, five or more rows of median ventral sets, from five to twelve slender 

 anal styles, and an uninterrupted border of marginal setae ; endoplast 

 single or multiple ; contractile vesicle usually situated near the left marginal 

 border. Inhabiting fresh water. 



This genus is distinguished from the Oxytrichidas previously enumerated by the 

 much more luxuriant development of the ventral setas. AVTiile in some species 

 there are not more than five rows of these elements, they very frequently clothe the 

 entire area of the ventral surface outside the peristome-field. 



Urostyla Weissii, Stein. 



Body elongate-elliptical, about three and a half times as long as broad, 

 widest centrally, gradually tapering towards the two extremities, the ante- 

 rior end the narrower ; peristome-field forming an acute triangle, extending 

 to a little beyond the anterior third of the body, its reflected border 

 * 'Zeitschrift fiir Wissenschaftliche Zoologie,' Bd. .\xxi., 1878. 



