248 ORDER FLAGELLATA-PANTOSTOMATA. 



Heteromita. A more careful investigation, however, aided by the employment of a 

 J— inch objective with a magnification of from 2500 to 5000 diameters, conclusively 

 demonstrated that no other ilagellate appendage existed, and that the single one 

 present fulfilled in a remarkable manner the functions performed in Hdcromita by two 

 such organs. In the free-swimming animalcules, which were less numerous than the 

 adhereni; ones, progression was efl'ected in a straight line, accompanied by an oscillat- 

 ino- motion, the single flagellum trailing in the rear like the posterior one oi Heteromita, 

 bnt slowly undulatmg throughout its length, and accomplishing by its vibrations the 

 advancement made. In the temporarily adherent forms, fixed to the glass object- 

 carrier or fragments of vegetable debris by the adhesive extremity of the same flagel- 

 lum, a similar undulating action of the remaining length of this organ was apparent, 

 this 'undulating action causing the entire organism to oscillate slowly up and down 

 (see PI. XIII. Fig. 50), and inducing at the same time a current to set in towards the 

 animalcule's body. Viewed in profile, the motile flagellum seen just beneath the 

 ventral surface of the body, presented at times an appearance closely corresponding 

 with that of a minute undulating membrane ; the body, however, in the next minute 

 tiltino- away from the flagellum, exhibited its true nature. The plienomena attend- 

 ino- the process of fission in this species were further observed to be somewhat 

 abnormal. This takes place obliquely, the first indication of the impending process 

 beino- a lengthening out of the body, accompanied by the greater prolongation of the 

 more or less pointed posterior extremity until it attains a curvature, though in a 

 reversed direction, corresponding with that of the anterior end, and develops at its 

 apex a flagellum similar in all ways to the anterior one originally possessed. No 

 trace of segmentation, however, has as yet made its appearance, and the animalcule 

 remains riding at anchor or floats through the water, presenting (as shown at 

 PI. XIII. Fio-. 51) a symmetrically sigmoidal contour closely identical with 

 that of Trepomonas agilis, as seen from a lateral point of view (see PI. XIX. 

 Fig. 11), the two similar flagella divergent from each recurved point assisting to 

 complete this likeness. Presently a faint obhque line makes its appearance, 

 extending from above the median point of the dorsal surface of the original 

 animalcule, downwards and backwards to behind the median point of the ventral 

 reo-ion. This faint line gradually increases in the clearness of its delineation, and 

 soon assumes the aspect of a distinct groove, which gradually deepens until the 

 anterior and posterior halves become separated from one another as two precisely 

 similar and undistinguishable units. Both bear the characteristic reflected flagellum, 

 and likewise the central- endoplast and antero-terminal contractile vesicle, these 

 respective structures having also made their appearance previous to the commence- 

 ment of the fissive process, the former by the segmentation of the original endoplast, 

 and the latter by independent development. The encystment of zooids which 

 previously exhibited an irregular amoeboid phase, and the subdivision of these into 

 eio-ht or sixteen macrospores, giving rise to animalcules similar in shape to, but of 

 much smaller size than the adults, have been observed, but not as yet the coal- 

 escence or genetic union of two or more units, and the breaking up of their united 

 masses into more minute and abundant microspores. 



ram. II. PLEUROMONADIDiE, S. K. 



Animalcules naked or illoricate, entirely free-swimming, flagellum single, 

 lateral or ventral ; no distinct oral aperture. 



Genus I. PLEUROMONAS, Perty. 



Animalcules free-swimming, kidney-shaped, bearing a single vibratile 

 flagellum which projects from the centre of the concave ventral side ; no 

 distinct oral aperture. 



