Unicellular, Microscopic Animals 349 



Reproduction. — The principal method of reproduction is by binary 

 fission, in which a full-sized Amoeba divides into two parts. The nucleus 

 divides by mitosis (Fig. 162) and the cytoplasm elongates and divides into 

 two parts. The entire process occurs in less than thirty minutes at 30° C. 

 After fission, the young amoebae grow rapidly, reaching mature sizes 

 about three days after fission. 



PARAMECIUM 



Paramecium (par ah -me' se um) (Gr. paramekes, oblong) is a large, 

 fresh-water protozoan commonly called the "slipper animalcule" because 

 of its fancied resemblance to a slipper. A common type, Paramecium 

 caudatum (ko-da'tum) (L. cauda, tail) has a tuft of tail-like cilia at 

 the posterior end and may be about 0.3 mm. long, while P. aurelia 

 (o-re'lia) (L. aurum, gold or brown) may be less than C.2 mm. long. 

 Most of the infusoria (in fu -so' ri a) (L. infusus, poured into or crowded) 

 possess (1) a large macronucleus (Gr. makros, large) composed of a 

 number of small, complete nuclei and (2) a small micronucleus (Gr. 

 mikros, small). P. caudatum has a blunt anterior end, and the posterior 

 end is somewhat pointed (Figs. 163 to 172). 



Integument (Covering). — Two types of cytoplasm, as in Amoeba, are 

 (1) an outer, clear layer, the ectoplasm, and (2) an inner endoplasm 

 with larger granules. A distinct pellicle (pel'ikel) (L. pellis, skin) 

 covers the ectoplasm. If a drop of 35 per cent alcohol is added to a 

 drop of Paramecium culture, the pellicle may be observed to separate 

 from the ectoplasm. Under high power, the pellicle is seen to be made 

 of six-sided hexagonal areas (noncellular) produced by ridges on the 

 surface (Fig. 170). One cilium projects from the center of each hexag- 

 onal area. The cilia arise from basal granules within the protoplasm 

 and are called microsomes ( mi' kro_^som ) (Gr. mikros, small; soma, 

 body). Spindle-shaped cavities called trichocysts (trik'osist) (Gr. thrix, 

 hair; kystis, sac) are embedded in the ectoplasm just beneath the surface. 

 These baglike cavities are filled with a liquid which solidifies into long, 

 jellylike threads when expelled to the exterior. Each trichocyst opens 

 through a pore on the ridges of the hexagonal areas. The poisonous 

 trichocyst fibers serve as weapons of defense. They may be observed to 

 discharge if a small amount of acetic acid is added to a drop of culture. 

 The basal granules are connected by longitudinal fibers. 



Motion and Locomotion. — Fine, hairlike cilia (sil'ia) (L. cilium, 

 hair) are regularly arranged over the entire external surface. These 



