ONE-CELLED ANIMALS OR PROTOZOA 49 



ticular spot of the surface where particles are 

 taken in. This may seem to be a restriction, 

 when we compare the Infusorian with Amceba, 

 which is apparently able to take in food at any 

 part of the surface. But it is a restriction which 

 is associated with an advantage ; the Infusorian 

 cell, namely, has a firm exterior with a definite 

 outline, instead of being soft and mobile all over. 

 The firmer exterior layer of protoplasm, which is 

 in turn covered by a thin cuticle or limiting mem- 

 brane, is called the cortex or rind. For this rea- 

 son the name Corticata is sometimes given to the 

 group, i.e., Protozoa with a rind. 



Vorticella, the Bell Animalcule, is a stalked 

 form living in ditches, which is usually selected as 

 a typical form of the Infusoria. It receives its 

 name, the Whirlpool Animal, from the current 

 which its cilia create in the water. The purpose 

 of this current is to wash food particles into the 

 mouth. Associated with the Infusoria under the 

 name of Corticata are the Gregarina and some 

 other parasitic forms. 



It is interesting to note that the main types of 

 the unicellular animals are repeated again in the 

 cells of different parts of the bodies of multi- 

 cellular animals. Amoeboid cells, so called be- 

 cause of their mobility and general resemblance 

 to Amceba, are found in various parts of the 

 higher animals. The lymph corpuscles of verte- 

 brata, and the white corpuscles of vertebrate 

 blood, as well as the blood corpuscles of inverte- 

 brates, are among the instances of this. There 

 are cells, on the contrary, such as those that line 

 the mucous tracts, which are of a Vorticella type, 

 so to speak; fixed to their bases, and presenting 

 cilia on the free aspect. 



