636 ORDER PERITRICHA. 



observed by the author that examples of this species can attach themselves to sur- 

 rounding objects by the distal end of their snout, which thus fulfils the purpose 

 of an adhesive sucker. The almost setose locomotive cirri, when such an attach- 

 ment is effected, quiver for a short interval with great rapidity, but finally become 

 entirely quiescent. The animalcules under these conditions, when seen from above, 

 may easily be mistaken for minute examples of the Radiolarian genus Actinophrys. 



The adherent snout in this type is, as in Didinium, capable of protrusion for 

 the seizure of prey to a considerable distance. When swimming, its locomotion is 

 rotatory, varied by occasional leaps from side to side after the manner of an Halteria. 

 This leaping motion is apparently effected by a similar sudden flexure of the loco- 

 motive cirri, which, while not so long and slender as in the last-named type, are 

 of an almost setose character. On a recent occasion the author noted, in an 

 animalcule apparently belonging to this species, what appeared to be a circlet of 

 minute vibratory cilia at the base of the conical snout, in addition to the normal 

 setose series, but this observation has not since been confirmed. 



Mesodinium pulex, C. & L. PL. XXXII. FIG. 44. 



Body turbinate, globose posteriorly, conical and tapering as it 

 approaches the anterior projecting snout ; wreath of cirrose cilia developed 

 on an annular groove or constriction half-way between the centre of the 

 animalcule's body and the base of the snout-like process. Length 

 of body 1-1650". HAB. Saltwater. 



This species, described by Claparede and Lachmann under the title of Halteria 

 pulex, is recognized by Stein as probably belonging to his genus Mesodinium. In 

 the diagnosis and description given by its discoverers it is represented as possessing 

 three long stylate processes, or bristle-like cirri, in advance of the mouth. Stein regards 

 these simply as three forward-projected locomotive cirri j but the present author is 

 inclined to accept them as an optical misinterpretation of the everted attenuate 

 proboscis, the two so-called outer styles representing the lateral walls, and the 

 central one the pharyngeal perforation of this structure. An almost identical 

 appearance is exhibited by Mesodinium acarus during the protrusion of this organ. 



GENUS IV. ACARELLA, Cohn. 



Animalcules free-swimming, more or less ovate, divided by an annular 

 constriction into a globose posterior and conical anterior portion, the 

 annular furrow bearing a wreath of long, slender, setose cilia ; the body 

 enclosed wholly or in part within an ovate or subglobular, transparent 

 sheath or lorica. 



The type form of this genus, as described by Cohn,* closely resembles the 

 species first described by Stein under the title of Mesodinium acarus, and also the 

 Halteria (Mesodinium) pulex of Claparede and Lachmann, but is of very much 

 smaller size, and has the posterior portion of its body enclosed within a transparent 

 lorica. 



Acarella siro, Cohn. PL. XXXII. FIG. 45. 



Body elongate-ovate, the posterior portion the larger, rounded and 

 subglobular, the anterior one conical, apically truncate ; setose cilia of 

 the intervening annular constriction equalling in length the larger posterior 



* " Neue Infusorien im Seeaquarium," 'Zeit. Wiss. Zool.,' Bd. xvi., 1866. 



