GENUS HALTERIA. 631 



ever, has elicited that these several genera, although diverging considerably from 

 one point of view, approach each other so closely when regarded from another, 

 while in many cases, again, so imperfect a knowledge is as yet possessed of their 

 more essential structural details, that it has been considered desirable for the present 

 to keep them associated. The most important structural deviations exhibited by the 

 members of this family, as here defined, is manifested by Halteria and Strombidium, 

 with their eccentric oral aperture and spirally involute adoral fringe, as compared 

 with the four remaining genera, in which the oral aperture is perfectly central, and 

 more or less disconnected from the ciliary system. Even here, however, Mesodinium 

 corresponds so closely in its general structure and comportment with Halteria, 

 that the marine species M. pulex was unhesitatingly referred by Claparede and 

 Lachmann to that genus. By Stein both Didinium and Mesodinium are separated as 

 types of a distinct family his Cyclodina; but, strangely enough, with these are 

 included by him an animalcule, Urocentrum turbo, with which they can scarcely 

 be said to possess a single point in common, and whose natural position is obviously 

 close to Gyrocoris, With but one or two exceptions all the members of the Hal- 

 teriidas are of exceedingly minute size, swift and restless in their movements, and 

 consequently form one of the as yet least perfectly investigated divisions of the 

 Peritrichous order. 



GENUS I. HALTERIA, Dujardin. 



Animalcules free-swimming, more or less globose ; oral aperture ter- 

 minal, eccentric, associated with a spiral or subcircular wreath of large 

 cirrose cilia ; a zone of long hair-like setae or springing-hairs developed 

 around the equatorial region, the sudden flexure of which appendages 

 enables the animalcules to progress through the water by a series of 

 leaping movements, in addition to their ordinary swimming motions. 



The animalcules of this genus are easily recognized by their globose form and 

 peculiar movements in the water, which consist of a slow rolling or rotatory 

 motion, interrupted at short intervals by a sudden leap backwards or to one side. It 

 was originally supposed by Dujardin that their springing motions were effected by 

 the contraction of the setse in connection with some fulcrum of support ; as, how- 

 ever, the same motions may be readily observed when the animalcules are swimming 

 freely at a distance from any foreign bodies, this hypothesis becomes untenable, 

 and it is evident that the leaps are produced simply by the sudden reflex action of the 

 hair-like setse. By the above authority this central girdle of setse is also represented 

 as following an oblique direction; such interpretation, however, represents an 

 erroneous optical impression frequently produced as the animalcules rotate through 

 the water at various angles divergent from their longitudinal axes. The exact 

 position of the oral aperture and relationship of the associated adoral cilia has not 

 up to the present time been decisively determined, the restless and rapid motions 

 of the animalcules rendering these two points especially difficult to interpret. By 

 many writers the oral opening is described as perfectly central, and the accom- 

 panying cilia as forming an unbroken circular wreath. Claparede and Lachmann, 

 however, go so far as to report the existence on one side of the peristome of a small 

 notch-like interruption. The author's impression, derived from repeated examina- 

 tions of the cosmopolitan form, H. grandinella, is that this adoral wreath is neither 

 symmetrically circular nor interrupted in the simple manner above indicated, but 

 that the right-hand limb or extremity of this wreath is, at the apparent notch, curled 

 spirally inwards, and there descends into the oral aperture. This interpretation, 

 reproduced in the accompanying illustration of the species quoted, demonstrates, if 

 correct, the character of the peristome or adoral fringe in Halteria to agree with 

 that of Strombidium and the Vorticellidae, and to be the converse of what obtains in 

 Stentor and other Heterotricha. In these, as explained in a previous page, it is the 

 left limb of the adoral fringe that spirally encircles and descends into the oral fossa. 



