THE CILIATA 217 



The general shape of the animal is that of a spindle 

 pointed at one extremity and rounded off at the other (fig. 

 47, A). It always moves with the blunt end foremost, so this 

 may at once be distinguished as the anterior, the opposite 

 pointed end as the posterior extremity. Further, it has a 

 definite ventral surface indicated by a groove which begins 

 as a shallow furrow at the anterior extremity and on the left, 

 and curves over with a slight spiral twist towards the right, 

 becoming narrower and deeper as it passes backwards. After 

 traversing rather more than the anterior half of the body, 

 the groove ends in the middle line in a relatively large and 

 deep funnel-shaped depression leading into the soft protoplasm 

 of which the centre of the body is composed. This depression 

 is the mouth, or, as some authors prefer to call it, the 

 cytostome, and the groove leading into it, the peristome. The 

 anterior half of the body is slightly twisted in connection with 

 the peristomial groove, somewhat as a leaden rifle-bullet is 

 twisted after it is fired through a grooved rifle-barrel; and 

 in consequence of this twist the animal spins round and 

 round on its long axis as it swims through the water, a 

 movement which makes it hard for a beginner to realise 

 its exact shape.* If it is killed Paramecium swells out and 

 loses its characteristic outlines. A living specimen, if its 

 movements are not artificially restricted, moves about with 

 great rapidity across the field of the microscope. It progresses 

 in a tolerably straight course with a uniform velocity, very 

 different to the jerky motions of a Flagellate. After going 

 for some distance in one direction it stops, turns, seems 

 to hesitate for a moment, and then darts off in a new 

 direction. 



These movements are entirely due to ciliary action. In a 

 specimen which is held fast by pressure, or, better, by being 

 placed in a thick solution of gum, the cilia can readily be 

 seen all round the margins of the animal. They are all of 

 equal size, except, perhaps, those at the posterior extremity, 



* Paramecium may easily be kept still by the use of a freshly-made 

 solution of gum-arabic. A few specimens should be placed in the smallest 

 possible drop of water on a glass slide. A droplet of methylene blue or 

 neutral red may be added to the water, and then a drop of fairly thick 

 gum. The coverslip is now put on, and the animals are kept nearly still 

 by the viscous mixture, but are not injured, and will live for some time 

 and gradually be stained by the dye. 



