Evolution 



animalcule, Paramoecium, shown in Fig. 23. This creature, barely 

 visible to the naked eye, is found in pools of water, or, for 

 example, in drops of rain or dew on plants, and it can generally 

 be obtained in great numbers by soaking a little hay in water for 

 a day or two. It has, as may be seen from the illustration, an 

 elongated shape, with a depression, the mouth, about the middle 

 of one side. The progress made good from the stage of the Amoeba 

 has been largely in the direction of a more efficient method of 

 locomotion. Instead of crawling, with painful slowness, the 

 Paramoecium swims freely and rapidly by means of the numerous 

 whip-like projections or cilia which cover it, and with which it 



\ 



EC 1 

 M 



FlG. 23. Parajnoecium. 



EC, Denser outer layer; EN, inner protoplasm; IV, nucleus; 

 PV^ contractile vacuole ; Af, mouth ; X, cilia. 



From Marshall and Hurst's Practical Zoology (Smith, Elder & Co.). 



lashes the water. An advance is also to be recognised in the 

 fact that the organism is surrounded by a dense outer wall ; and 

 that its shape is consequently fixed. Hence also the Paramoecium 

 cannot take in food at any part of its surface, as the Amoeba can, 

 but only through the special depression already mentioned. 

 Excretion is carried on in the same manner as in the Amoeba. 

 The Paramoecium is a water animal, yet it can resist drying, and 

 remain alive in the absence of water, for a long period This it 

 accomplishes by becoming encysted, that is, by contracting into a 

 ball and surrounding itself with a resistant shell, from which it 

 can emerge when suitable conditions for active life return. It 

 is worth passing notice that there exist a number of forms occupy- 

 ing, a position intermediate between the two types which we have 



32 



