GENUS ANISONEMA. 435 



usually considerably thicker towards the base ; oral aperture adjacent to 

 the origin of the posterior flagellum, communicating with a short, tubular 

 pharyngeal tract ; contractile vesicle single, situated behind or a little to 

 the left of the oral aperture ; endoplast spherical or ovate, located on the 

 right-hand side towards the posterior extremity. Length 1-1250" to 1-900". 

 HAB. Pond water, amongst Conferva. 



This typical species of the genus, more generally known by Dujardin's name of 

 Anisonema acinus, and recently figured and described by Professor Clark as Anisonema 

 concavum, is identified by Stein with the Bodo grandis of Ehrenberg, whose primarily 

 conferred specific title necessarily takes the precedence of all others. The motions 

 of this animalcule in the water, typifying also those of the genus generally, are very 

 characteristic, but at the same time closely resemble those of the ordinary members 

 of the genus Heteromita previously described. Progress through the water consists 

 chiefly of a smooth, gliding motion in a straight line, the shorter anterior flagellum, 

 or tractellum, being projected and vibrating in advance, while the long posterior one, 

 or gubernaculum, as it is designated by Professor Clark, trails cable-wise, like a 

 boat dragging its anchor, in the rear. This even locomotion is occasionally varied 

 by a laborious hitching gait, accompanied by a swinging of the animalcule from 

 side to side, while now and then the body is brought up sharply and propelled in 

 an opposite direction, through the adhesion to the ground, and simultaneous 

 loosely spiral contraction, of the trailing gubernaculum. The locality proving 

 auspicious and the food-supply plentiful, the little animalcule often fixes itself by 

 the distal extremity of the last-named organ and rides gracefully at anchor, fishing 

 vigorously with its extended tractellum, detaching itself after a short interval, and 

 progressing as before in search of a new pasture-ground. 



The aspect of this species as seen in profile is subject to considerable variation 

 according to the point of view taken, and in consequence of the asymmetrical 

 development of the ventral excavation. Its most characteristic concavo-convex or 

 meniscoid contour is best illustrated when seen end-on from behind, as represented 

 at PI. XXIV. Fig. 26. The ventral excavation, so conspicuous under these con- 

 ditions, is shown to be abruptly and obliquely deepened on its left-hand border, 

 which thus appears to be recurved over it, while it narrows off gradually on the 

 right-hand one. It is this character of the ventral furrow that imparts to the 

 animalcule a meniscoid contour as seen only from the right-hand side, a view from 

 the opposite or left-hand one yielding the simpler plano-convex shape that has been 

 more frequently ascribed to it. As shown by Biitschli, individuals vary somewhat 

 in their dorsal contour, the ovate or " pip-shaped " outline being sometimes 

 replaced by a shorter, broader, and almost elliptical form. Increase by longitudinal 

 fission has been observed in this species by various investigators, the two flagella 

 destined for the new zooid often being developed at the anterior extremity of 

 the primary individuals prior to the commencement of the dividing process. At 

 this stage, PL XXIV. Fig. 27, the animalcule has the appearance of possessing 

 two or three vibratile anterior flagella, and a single or double trailing one. De 

 Fromentel,* supposing it to represent a new type, has figured and described this 

 transitional condition under the title of Diplomita insignis, while he confers that of 

 Heteromita crassa upon the ordinary biflagellate condition of the same animalcule, f 

 Stein, in his recently published volume, \ represents the anteriorly located con- 

 tractile vesicle as having frequently a rosette-like aspect through the association of 

 smaller lateral lacunae, and also the development, within the central endoplasm, of 



* ' Etudes sur les Microzoaires,' Paris, 1876. 



t This earlier though fruitless employment of the term Diplomita by De Fromentel had escaped 

 the author's attention when proposing the same title for the biflagellate animalcule described at 

 p. 289. It being desirable to substitute a new generic name for this altogether distinct form, that of 

 Difloselmis is herewith given in exchange. 



J ' Infusionsthiere,' Abth. iii., 1878. 



2 F 2 



