46 MANUAL OF ZOOLOGY SECT, i 



trichocysts (trch.\ When the animal is irritated, more or 

 fewer of these suddenly discharge a long delicate thread 

 (C), which, in the condition of rest, is very probably coiled 

 up within the sac. 



Food, in the form of small living organisms, is taken in 

 by means of the current caused by the cilia of the buccal 

 groove. The food-particles, enclosed in a globule of water, 

 or " food-vacuole " (f. vac.\ circulate through the proto- 

 plasm, and the soluble parts are gradually digested and 

 assimilated. Effete matters are egested at a definite anal 

 spot posterior to the mouth, where the cortex and cuticle 

 are less resistent than elsewhere. The whole feeding pro- 

 cess can readily be observed in this and other Infusoria by 

 placing in the water some insoluble colouring matter, such 

 as carmine or indigo, in a fine state of division, the 

 minute particles of the colouring matter, which are taken 

 into the mouth in the way described, being readily observed 

 as they become received into food-vacuoles and circulate in 

 the central protoplasm. 



Multiplication takes place by transverse fission (D), the 

 division of the body being preceded by that of both nuclei. 

 It has been proved, however, that multiplication by binary 

 fission cannot go on indefinitely, but that after it has been 

 repeated a certain number of times, it is interrupted by 

 conjugation. In this very remarkable and characteristic 

 process two Paramcecia become applied by their ventral 

 faces, but do not fuse ; their meganuclei break up and dis- 

 appear, and an interchange of the substance of the micro- 

 nuclei of the two conjugating individuals takes place, with 

 the result that each develops a new meganucleus and a 

 new micronucleus, partly formed of the substance of its own 

 micronucleus, partly of that of the other Paramcecium. 



The possession of cilia is the distinctive feature of the 



