THE SIMPLEST ANIMALS OR PROTOZOA 25 



esting additional characters. For example, the form repre- 

 sented in Fig. 7, C, constructs a sac-like skeleton of tiny 

 pebbles cemented together, into which it may withdraw for 

 protection. Others construct similar envelopes of lime or 

 flint, and still others, as they continue to grow, build on 

 additional chambers, giving rise to a great variety of forms 

 often of wonderful beauty. In the tropics, particularly, 

 some of the shelled Protozoa are so abundant that they may 

 impart a whitish tinge to the' water, and in some places 

 their empty shells on falling tci the bottom form immense 

 deposits. The chalk cliffs of England are in large measure 

 made up of such shells. 



25. The Infusoria. A little over two hundred years ago 

 it was discovered that wherever water remained stagnant it 

 became favorable for the rapid multiplication of a large 

 number of species of Protozoa which live in such situations. 

 These are known as Infusoria, and, like the preceding spe- 

 cies, are usually of microscopic size and of the most varied 

 shapes. The first striking feature of their organization is 

 the' presence of a delicate though relatively firm external 

 cell membrane known as the cuticle, which preserves a defi- 

 nite shape to the body. Such a method of locomotion as 

 exists in the preceding group is consequently an impossi- 

 bility, but other and more highly developed structures per- 

 form the office. These latter organs are of two types, and 

 their general characteristics may be readily understood 

 from an examination of a few species living in the same 

 localities as the Amceba. 



26. The Euglena. The first type exists in the common 

 fresh-water organism known as Euglena, represented in 

 Fig. 8, A. Here the spindle-shaped body is surrounded by 

 a delicate cuticle perforated at one point, where a funnel- 

 shaped depression, the gullet, leads into the soft proto- 

 plasmic interior. From the base of this depression the 

 protoplasm is drawn out in the form of a delicate whip-like 

 process known as the flagellum. This structure, always 



3 



