136 GENERAL HISTORY OF THE INFUSORIA. 



and other Diatomeae, in which both the branched stem and the beings it 

 supports are ahke part and parcel of the same organic structni'e. He has 

 met with fibres supporting but one or two Uvella -bundles, and others like 

 little trees bearing ten such. The consistence of the stem is such that it 

 resists the action both of sulphuiic acid and of solution of potash. 



One other genus oi Monadina, viz. Polytoma (XVIII. 5), has received special 

 attention from Schneider, Cohn, and Perty ; it nevertheless still remains in 

 that neutral ground claimed both by zoologists and botanists. Ehrenberg at 

 first placed it in the family Monadina ; but having subsequently met with a 

 similar form, Sponclylomorum, he instituted a new family, Uydromorina, to 

 include the two genera, and set forth as its chief differential characters the 

 aggregate or compound nature of its members, dependent on imperfect fission. 

 He asserted also that individuals set free from the groups enter on the same 

 cycle of fission and compound development, and form similar groups. Foly- 

 toma was described to be destitute of an eye-speck, to have a truncated 

 mouth and a dehcate double flagelliform proboscis, and, from repeated incom- 

 plete self- division, to form a mulberry-like mass, which eventually breaks up 

 into isolated Monads. " The ova," he adds, " from their minuteness and the 

 want of transparency, have hitherto eluded observation (XYIII. 5) : but the 

 alimentary organization is, on the contrary, clearly demonstrable ; for al- 

 though for a long time the entrance of coloured food could not be displayed, 

 yet at length, by using a magnifying power of 600 to 800 diameters, the 

 entrance of indigo-particles into their bodies was rendered evident." In 

 addition to these structures, he mentions a large contractile vesicle as a male 

 sexual organ, and a white spot at the anterior part of the body as a seminal 

 gland. In all essential particulars the associated genus Spondylomorum was 

 stated to agree with it, except in having a dorsal eye- speck. 



Dujardin confesses his inability to distinguish by any definite characters 

 between Uvella and Polytoma ; he would seem, however, not to have per- 

 sonally investigated the latter. Cohn, after examining both, declares them 

 to be identical in all particulars except that in Polytoma chlorophyll is absent, 

 and that it inhabits decomposing fluids along with Chlamydommias j)ulviscu- 

 lus. However, it is to Schneider that we are indebted for the most complete 

 history of this organism (Inaugural Dissertation, '' Symholce ad Infusorium 

 historiam naturalem,'" Berlin, 1853, translated in A. N. H. 1854, xiv. p. 321). 

 We extract the foUo^dng copious details from the translation : — " Polytoma 

 Uvella is of an oval form ; it is from -g^th to ^\j-th of an inch long, and about 

 half that width. At one end, which, mth Ehrenberg, we will call the an- 

 terior extremity, it bears two filaments as long or longer than the body. 

 "When the living animal is examined under a magnifying power of 300 dia- 

 meters, the body appears to be bounded by a simple outhne. But in many 

 instances, and especially when a large specimen can be found at rest, it may 

 be seen that the internal substance of the body is surrounded by a thin and 

 perfectly clear membrane, from which it is separated by a distinct space. 

 When the investing membrane is more closely attached, its existence may 

 always be demonstrated by the employment of reagents to produce the con- 

 traction of the substance of the body : chromic acid and solution of iodine in 

 chloride of zinc are the best substances to employ, the latter especially, as it 

 at the same time communicates a brown colour to the internal sac (PI. XX. 

 fig. 2). Under certain circumstances, the investing membrane divides into 

 minute granules, assuming when viewed from the side a regular necklace- 

 like appearance (fig. 8). A reproduction of the membrane then takes place. 

 The substance of the body is perfectly clear, with the same refractive proper- 

 ties as that of Amwha. About the middle lies a clear globular nucleus, sur- 



