^G ELEMENTS OF BIOLOGY 



through obstructions, or changes direction suddenly, its shape is 

 constant. In addition to the ciha, which form part o£ the mechanism 

 o£ food taking as well as locomotion, the animal is covered with a 

 non-living membrane or pellicle, under which is the ectosarc. The 

 cilia constitute an example of specialization of protoplasm, for these 

 structures exaggerate the function of contractility and carry on for 

 the remainder of the cell, which in general is much less contractile 

 than amoeba. In the ectosarc is also to be found another specialized 

 system of structures, the trychocysts. Ordinarily these are not 

 visible under the lower powers of the microscope, but the para- 

 moecium may be induced to discharge them if it is brought into 

 contact with a bit of acetic acid or other irritant. The appearance 

 of the discharged trychocysts is that of a fine fibre entanglement 

 forming a sort of defending net, thus serving to keep an enemy 

 at a distance. 



The cytoplasm of the animal is constantly circulating, a phe- 

 nomenon known as cyclosis (see p. 58), and usually contains num- 

 bers of food vacuoles. The nuclear parts consist of two structures, 

 a large macronucleus, closely adjacent to which is a much smaller 

 MiCRONUCLEus. There are also two contractile vacuoles, one in the 

 forward and one in the rear region. 



Nutrition in Paramoecium. The paramoecium almost invari- 

 ably moves with its sharpest end as the leading element and the 

 cilia are so coordinated as to direct this movement. Its stream-lined 

 shape facilitates motion through the water. A distinct groove, called 

 the ORAL GROOVE, occurs on one aspect of the animal, so placed that 

 it opens forward. This groove is lined with cilia somewhat longer 

 than usual, and in it is suspended a thin membrane, the undulating 

 MEMBRANE. The cilia and the membrane by their motions sweep 

 water containing bacteria and other bits of food material down 

 the oral groove and into a short passage, the gullet, at the base 

 of which is a region of ectosarc which is specialized to engulf the 

 food particles in a manner similar to the taking in of food in 



