374 ORDER FLAGELLATA-EUSTOMATA. 



in length ; endoplast subcentral ; contractile vesicle situated towards the 

 anterior extremity, and to one side of the pharynx. Length 1-500". 

 HAB. Fresh water. 



The animalcules of this species, in addition to swimming freely in the water, 

 creep over the surfaces of submerged objects, with their oral region applied to the 

 same, much in the same manner as Atractonema. An approach to the repent 

 mode of progression exhibited by these two last-described generic forms would 

 appear to be foreshadowed in the species provisionally referred by the author to the 

 genus Petalomonas, under the name of P. irregularis, and in which the distal ex- 

 tremity of the flagellum persistently maintains a close relationship with the surface 

 of the ground or objects traversed. 



GENUS V. MENOIDIUM, Perty. 



Animalcules free-swimming, persistent in form, lunate or ensiform, com- 

 pressed, bearing a single terminal flagellum, at the base of which is situated 

 the oral aperture, followed by a minute tubular pharynx ; endoplasm trans- 

 parent, granular ; contractile vesicle and endoplast conspicuous, the former 

 situated close to the termination of the pharynx. Inhabiting fresh water ; 

 movements oscillating or rotatory. 



Menoidium pellucidium, Pty. PL. XX. FIGS. 15 AND 16. 



Body lunate, compressed, four to six times as long as broad, with a 

 more thickened, convex ventral, and concave dorsal border, most attenuate 

 anteriorly, its superior edge sometimes developed above the oral aperture as 

 a projecting tooth-like spine ; flagellum slender, equalling the body in 

 length ; endoplasm transparent, more or less granular, usually containing 

 one or more large ovate amylaceous corpuscles ; contractile vesicle, appa- 

 rently communicating with the termination of the pharynx ; endoplast 

 subcentral. Length 1-600" to 1-400". HAB. Fresh water. 



The general form of the body of this animalcule may be compared to that of a 

 minute transparent Closterium. As originally figured and described by Perty, the 

 terminal flagellum is not represented, and its existence only suspected. Stein * is 

 the first who has published an amplified delineation of its contour and component 

 structure, the chief details of which the author is in a position to corroborate, 

 having recently, November 1878, encountered the species in marsh water obtained 

 from the neighbourhood of St. Heliers, Jersey. In none of the examples as yet 

 examined, however, was detected the anterior tooth-like projection characteristic 

 of the majority of Stein's figures, while in all cases the endoplasm exhibited a feature 

 previously unnoticed. In Stein's delineations the granulation of this element 

 presents no definite plan of distribution, and is diversely developed in different indi- 

 viduals. In those personally investigated, the granulation was confined entirely to 

 the posterior region of the body, and consisted of innumerable minute spherical 

 corpuscles of uniform size, as shown at PI. XX. Fig. 15. In most of these there 

 was a single large, elliptical, subcentral, amylaceous corpuscle ; but in some instances, 

 as in Stein's figures, two or more smaller ones. An exceptional specimen exhibited 

 an inflated and almost cylindrical in place of the usually compressed body contour. 

 Dead examples occasionally occur in which the flagellum has disappeared, but the 

 animalcule still retains its distinct tubular pharynx, thus demonstrating the indurated 

 character of the walls of this structure. 



* ' Infusionsthiere," Abth. Hi., 1878. 



