FLAGELLATE INFUSORIA AND ALLIED ORGAN ISMS. 97 



single colony. The yellowish-green colouration is depen- 

 dentj as in other cases, upon the presence of two pigment 

 discs, which are relatively not very thick, of which one 

 occupies each lateral half of the body, lying close beneath 

 the surface. Between the two discs is only a small space, 

 which appears on closer inspection as a colourless trans- 

 parent line running longitudinally. The pigment is, there- 

 fore, not present in the body-substance in a finely divided 

 condition, as Stein has described in certain red and yellow 

 Flagellata. The pigment discs of Uvella are remarkably dis- 

 tinct ; after the death of the organism, when the body protoplasm 

 swells up, and the whole body becomes rounded, the pigment 

 discs undergo the same change, and ultimately shrinking to- 

 gether form irregular or roundish bodies (fig. 22, b and c) . In 

 the hinder end of the body, at the spot where the tapering gives 

 rise to the colourless point, are found two small contractile 

 vacuoles close to each other (fig. 22, a v), which contract al- 

 ternately and reappear upon the same spot. A nucleus cannot 

 be seen under the ordinary conditions, although it can be 

 detected by staining. As demonstrated by Beale's carmine, 

 it lies somewhat in the centre of the body between the two 

 pigment discs, whilst the rest of the body remains quite 

 uncoloured {71 in fig. 22, b and c). The body protoplasm 

 is generally filled either with fine or coarse granules in 

 larger or smaller numbers ; the anterior end alone often 

 appears to be quite free from such granules, and is 

 therefore transparent. Only a single stage in the process 

 of fission has been observed (fig. 22, d), and it agi-ees with 

 the usual mode followed by other Flagellata. The division 

 occurs lengthwise, and then the flagella and contractile 

 vacuoles increase in number, as do also the pigment discs 

 (fig. 22, d). 



Encystation has also been observed in Uvella ; such en- 

 cysted forms being often found on crushing a colony among 

 normal organisms, and also lying free in the watch glass in 

 which the Uvella has been kept. These forms consist of a 

 delicate irregular outer coat, enclosing a thicker coat, which 

 immediately surrounds the encysted body. Within the cyst 

 lie the much contracted pigment discs. 



Among the individuals forming the Uvella colony a small 

 Flagellate is found so frequently as to be almost normal 

 (fig. 22, a). This small organism is long and spindle- 

 shaped, it possesses two flagella, and fixes itself with its 

 flagella-bearing end — which is more pointed — towards the 

 centre of the colony. It calls to mind Chlora<jomum 



VOL. XIX.— NEW SER. G 



