114 



THE RISE OF ANIMAL LIFE 



Fig. 7-6. Paramecium has been studied perhaps more 

 than any other protozoan. It is large (100-200 mi- 

 crons in length) and very easily cultured In the 

 laboratory. 



If it is floating free in the water and one of 

 its pseiidopods contacts a surface, it at once 

 adheres to the substratum. However, if a 

 sharp point is pressed into its protoplasm 

 at the surface, an immediate avoiding reac- 

 tion follows. Amoeba thrives best at room 

 temperature. If it is subjected to lower tem- 

 peratures, all of its activities slow down and 

 cease altogether as the freezing point is 

 reached. Activity also ceases if the tempera- 

 ture is raised to approximately 30° C. 



In general, then, the behavior of amoeba 

 is geared to its needs and is responsible for 

 its survival. If more were known about the 

 amoeba's response to its external world, 

 problems which arise in the more special- 

 ized animals would be easier to solve. 



PARAMECIUM 



Not all Protozoa are as simple as amoeba. 

 A study may now be made of a protozoan 

 which is perhaps one of the most compli- 

 cated of all single cells — paramecium, the 

 "slipper animalcule" (Fig. 7-6). This form 

 has been experimented upon and studied 

 as much, if not more, than amoeba and a 



great deal of fundamental biological infor- 

 mation has been derived from this source. 

 As one might guess, the animal is shaped 

 like a slipper, pointed at the posterior end 

 and blunt at the anterior end (Fig. 7-7). 

 A groove extends throughout most of its 

 length and the mouth or cytostome (cell 

 mouth) is formed in the groove on the 

 ventral side, about two-thirds back from 

 the anterior end. Paramecium has an outer 

 covering, the pellicle, which is sufficiently 

 rigid to maintain a constant shape. Careful 

 examination with excellent optical equip- 

 ment shows that the covering is made of 

 minute hexagonal plates, and that the mid- 

 dle of each plate is perforated by a central 

 opening through which a tiny "hair-like" 

 cilium (plural, cilia) passes. The animal 

 moves by the combined rhythmic beating 

 of these cilia. At the junction of the plates 

 are other tiny holes through which threads 

 are thrust when the animal is disturbed. 

 The threads originate from small bodies ly- 

 ing just beneath the pellicle, called tricho- 

 cysts. They are apparently used in defense 

 and perhaps also in attaching the animal to 

 detritus in the water. Adding a small 

 amount of acetic acid to the water near the 

 Paramecium discharges them. 



Locomotion 



The power stroke of the cilia is diagonal, 

 so that the animal turns on its long axis. 

 Since the cilia in the oral groove are larger 

 and beat with more vigor, the anterior end 

 describes a circle and causes the animal to 

 swim in a spiral manner (Fig. 7-8). When 

 the posterior end is stationary, the long axis 

 of the body describes a cone (Fig. 7-9). 

 When the animal is confronted with an 

 obstacle, the cilia reverse their effective 

 beat so that the cell moves backward a 

 short distance, turns slightly, then moves 

 forward again. If it meets the obstacle 

 again, the process is repeated until the ani- 

 mal passes around and goes on its way 

 (Fig. 7-10). This is known as an avoiding 

 reaction. 



