infusoria: — contractile vesicles. 457 



pellets that first entered make their way out after a time (after 

 yielding up their nutritive materials), generally by a distinct anal 

 orifice, sometimes, however, by any part of the surface indiffer- 

 ently, and sometimes by the mouth. A circumstance which seems 

 clearly to indicate that they cannot be enclosed (as maintained by 

 Prof. Ehrenberg) in distinct stomachal cavities, is that, when the 

 pellets are thus moving round the body of the Animalcule, two of 

 them sometimes appear to become fused together, so that they 

 obviously cannot have been separated by any membranous invest- 

 ment. When the Animalcule has not taken food for some time, 

 ' vacuoles, ' or clear spaces, extremely variable both in size and 

 number, filled only with a very transparent fluid, are often seen in 

 its sarcode ; their fluid sometimes shows a tinge of colour, and 

 this seems to be due to the solution of some of the vegetable chlo- 

 rophyll upon which the Animalcule may have fed last. 



342. Contractile Vesicles (Fig. 238, a, a), usually about the size 

 of the ' vacuoles,' are found, either singly or to the number of from 

 two to sixteen, in the bodies of most Animalcules ; and may be 

 seen to execute rhythmical movements of contraction and dilata- 

 tion at tolerably-regular intervals, being so completely obliterated 

 when emptied of their contents as to be quite indistinguishable, 

 and coming into view again as they are refilled. These vesicles 

 do not change their position in the individual, and they are pretty 

 constant, both as to size and place, in different individuals of the 

 same species ; hence they are obviously quite different in character 

 from the 'vacuoles.' In Paramecium there are always to be 

 observed two globular vesicles (Fig. 238, b, a, a), each of them 

 surrounded by several elongated cavities, arranged in a radiating 

 manner, so as to give to the whole somewhat of a star-like aspect 

 (Plate xiv., fig. 1, v, v); and the liquid contents are seen to be pro- 

 pelled from the former into the latter, and vice versa. Further, 

 in Stentor, a complicated network of canals, apparently in con- 

 nection with the contractile vesicles, has been detected in the sub- 

 stance of the ' cortical layer ;' and traces of this may be observed 

 in other Infusoria. In some of the larger Animalcules it may be 

 distinctly seen that the Contractile Vesicles have permanent val- 

 vular orifices opening outwards, and that an expulsion of fluid from 

 the body into the water around is effected by their contraction. 

 Hence it appears likely that their function is of a Respiratory 

 nature, and that they serve, like the gill-openings of Fishes, for the 

 expulsion of water which has been taken-in by the mouth, and 

 which has traversed the interior of the body. (See § 329.) 



343. Of the Reproduction of the Infusoria our knowledge has 

 lately received a great accession in the discovery of their true 

 Sexual Generation (§ 347); the attention of observers having, until 

 a comparatively recent period, been fixed almost exclusively upon 

 the act of Binary Subdivision, which, though by far the most 

 frequent method of propagation, is not a true generative operation. 



