BRITISH rHESnWATER EHIZOPODA. 221 



from that of Amoeba proteus or A. villosa. It attracts attention 

 at once by its strikingly refringent ectoplasm. Besides this dis- 

 tinctive feature the organism is remarkable for the possession of 

 an anterior flagellum, 0-06 to 0*08 mm. long, according to 

 Hertwig and Lesser, which these authors rightly describe as 

 " a very fine filament of equable but hardly definable character, 

 and considerable refractive power." It is "not diminished at the 

 apex, but ends as if abruptly cut ,off." In all the Cheshire 

 examples met with this organ was so transparent and so fine 

 as to be. with difliculty distinguished; in some it was imper- 

 ceptible; but in every case, where seen, it was in a constant 

 state of activity, so much so that it was impossible to bring 

 it into focus at once throughout its whole length. The organ 

 was not used for purposes of locomotion. The movements of 

 the animal are truly amoeboid; although very jj'oung and small 

 examples were seen to effect a slight jerky movement from side 

 to side. 



Usually two contractile vacuoles may be seen embedded in the 

 densely granular endoplasm, in the posterior region — one on each 

 side of the body. The entire body-surface has the remarkable 

 peculiarity, in adult individuals, of being covered, more or less, 

 with a thin stratum of extremely minute bacilliform bodies, 

 probably siliceous, and secreted during growth. Sometimes 

 these are hardly perceptible, and in young individuals they are 

 either absent or few in number. 



The body of Mastigamoeba aspera often assumes an irregularly 

 spherical or elliptical shape, with radiating pseudopodia of 

 Tariable length, in which condition the animal presents a totally 

 different appearance from that of the same individual en tnarche. 

 It then closely resembles the figures of Dactylosphcdrium vitreum. 

 The resemblance, indeed (aided by the refringent character of the 

 protoplasm), is so striking, that one Can hardly avoid the con- 

 clusion, as was long ago suspected, that the two are phases of 

 the same organism. In what may, for convenience, be called the 

 D. vitreum phase, Mastigamceba aspera never exhibits a flagellum. 

 An individual is figured (fig. 4) in which, though here concealed, 

 this organ was distinctly visible so long as the animal was in its 

 normal condition — that is to say, en marche. Frequently small 

 and apparently young examples are met with which preserve 

 the D. vitreum form, and exhibit only superficial changes, for 

 considerable periods. In recovering from this condition the 



