FLAG3LLATE INFUSORIA AND ALLIED ORGANISMS. 85 



VI, fig. 17, a, c), as the end which is devoid of flagella 

 is sometimes pointed, sometimes rounded, and sometimes 

 is bent into a sharp hook-shape, and again, may be 

 without any such bend. The colourless examples from in- 

 fusions are generally small, about 0*022 mm., whilst the 

 brown forms from the ponds are, for the most part, propor- 

 tionately very large, attaining a length of 0*049 mm. ; others, 

 however, only attain a length of 0"015 mm., and conse- 

 quently, are very small. There is so little variety amongst 

 them, that the author believes, with Perty, that they are all 

 of one species. The anterior end is considerably wider than 

 the posterior, and the lip is well marked, although it is so 

 far back that the cleft shows but little at the anterior end. 

 The two flagella in front are of equal length and strength. 

 The strength of the flagella is considerable and does not 

 lessen towards the end. The deportment of these flagella 

 during the state of rest is very peculiar, for the organisms 

 frequently tuck them in, rest for a considerable time, and 

 then make them suddenly revolve with great rapidity. The 

 two flagella frequently assume during the resting state the 

 positions shown in fig, 17, c, but they are also frequently 

 seen in very cramped positions. The point of origin of the 

 two flagella is not accurately determined, but they appear 

 to arise at some distance from each other, probably one from 

 the upper and the other from the lower lip, as is figured in 

 fig. 17, c. The cytostome (mouth opening) is situated 

 between the two lips. It leads into a short tube which 

 is quite transparent, and which is bounded by thin walls ; 

 this leads into the gullet provided with thick and dark walls 

 (fig. 17, a, and c, oe). This cavity, as seen from the surface, 

 appears to be striped longitudinally as well as transversely, 

 so that a. knotted appearance is seen at the points where the 

 two systems cross each other ; the walls, therefore, in optical 

 section, appear to be knotted (fig. 17, a). The substance of 

 which these walls are composed seems to be thickened pro- 

 toplasm, because they do not remain after the death of the 

 animal, but are destroyed. The ingestion of food has not 

 been satisfactorily followed out, but they appear to take in 

 food in the same way as do the other Flagellata. 



The contractile vacuole is situated in the upper lip, where 

 it is easily visible ; the contractions occurring but slowly, 

 it requires attention to observe them ; this, however. Stein 

 has accomplished.^ 



In the coloured varieties, the pigment is by no means 



^ Fr. Stein, 'Die infusionsthiere auf ihre Entwickelungsgeschichte 

 untersucht.,' Leipzig, 1854, i, p. 91. 



