30 



PRINCIPLES OF ANIMAL BIOLOGY 



bine to produce cells of a great variety of shapes. They may be rather 

 regular rectahedrons, as in plants; long cylinders, as in voluntary muscle; 



Fig. 17. — Two protozoa of the class Rhizopoda. The bodies are partly enclosed 

 in shells, one made of sand, one of a chitinous substance. From these shells the proto- 

 plasmic body may be actively protruded in arm-like pseudopodia, which are afterwards 

 retracted. 





Fig. 18. — Various forms of Infusoria whose body shape is kept fairly constant by a 

 surrounding pellicle. Though this shape may be altered by pressure, it is restored when 

 the pressure is removed. Cilia project from the surface. (Modified from Conn.) 



greatly flattened cells with their largest sides polygons, as in the outer 

 layer of frog skin; somewhat flattened cells elliptical on their flat sides, 

 as in the blood of many animals; circular and flattened or cupshaped, as 



