36 



PROTOZOA 



[CH. 



FIG. 11. Vorticella 

 microstoma x about 



200. 

 From Stein. 



and fro, these are the cilia ; there is a circle, or rather one twist of 

 a spiral of these, as we can see when the animal 

 turns the surface of the disc upwards. By the 

 regular rhythmical bending of these cilia, and 

 possibly by the movement of a rolled membrane 

 which projects into the mouth, a vortex is pro- 

 duced in the water, which draws particles of 

 food to the Vorticella, as is the case with the 

 higher Flagellata and Sporozoa. The shape of 

 the body, though it varies slightly with the 

 state of expansion or contraction, is practically 

 constant ; no pseudopodia are given out. This 

 is the result partly of the possession of a 

 firm membrane covering the whole of the body, 

 called the cuticle, which is a protective secre- 

 tion like the shell of Arcella, only much 

 thinner and so intimately connected with the 

 protoplasm under it as to be inseparable from 

 it; but the constancy of shape is also due to the fact that as in 

 Sporozoa or Flagellata the ectoplasm is a cortical layer con- 

 taining myonemes or fibrils endowed with the power of con- 

 traction. The stalk, which is entirely composed of this layer, might 

 almost be regarded as a muscular fibre. The stalk is slightly 

 twisted and attached in a long spiral to the inner side of an elastic 

 tube of cuticle. When contraction occurs the stalk is necessarily 

 thrown into the most evident spiral curves, like a corkscrew; the 

 restoration of the form after contraction is due to the elasticity of 

 the tube of cuticle. 



Vorticella possesses a contractile vacuole and a nucleus just as 

 Amoeba does ; in small nearly transparent specimens both are easily 

 detected during life ; in fact, if the specimen under observation only 

 keeps moderately still, we can follow the expansion and contraction 

 of the vacuole with the greatest ease. The nucleus is very large 

 and has more or less the shape of a horse-shoe, though the two ends 

 are generally at different levels, so that in reality it forms part of a 

 spiral. If we run in some iodine it at once absorbs the stain and 

 stands out very distinctly ; the Vorticella, however, frequently shows 

 its dislike to the operation by contracting its body into the shape of 

 a ball and snapping itself off from the stalk : it is then apt to get 

 washed away from its position by the inflowing iodine and we may 

 have to search over the slide to find it. This conspicuous nucleus 



