THE PROTOZOA 17 



are growing together they appear like a delicate growth 

 of mold upon the water weed. The stalk is peculiar in 

 being traversed by a muscle fiber arranged in a loose spiral, 

 which upon any unusual disturbance contracts together 

 with the body into the form shown in Fig. 8, b. 



These few examples serve to show the general plan of 

 organization and the method of locomotion of the Infuso- 

 ria ; but, as upward of a thousand species exist, with widely 

 differing habits, many interesting modifications are present. 

 Some have been driven in past time to adopt a parasitic 

 mode of life within the bodies of other animals. At pres- 

 ent they are devoid of locomotor organs, and as they absorb- 

 nutritive fluids through the surface of the body all traces 

 of a mouth are also absent. The reproductive processes 

 also are peculiar, but they do not concern us now. 



20. Characteristics common to the Protozoa. We have 

 now studied the principal structures which serve in loco- 

 motion among these simple one-celled forms, also the means 

 by which they catch their food, and we shall now glance at 

 the internal processes, Avhich are much the same in all. 



After the food has been taken into the cell, it is prob- 

 ably acted upon by some digestive fluid, for it soon assumes 

 a granular appearance and finally undergoes complete solu- 

 tion. In every case the oxygen is absorbed through the 

 general surface of the body, and uniting with the living 

 substance, as in the squirrel, liberates the energy necessary 

 for the performance of the animal's life work. The wastes 

 thus produced in a large number of forms simply filter out 

 from the body without the agency of anything comparable 

 to a kidney, but in several species they are borne to a 

 definite spot, the pulsating vacuole (Figs. 5, 7, 8, c.v.), where 

 they gradually accumulate into a drop about the size of the 

 nucleus. The wall between it and the exterior now gives 

 way and the excretions are passed out. In active indi- 

 viduals this process may be repeated two or three times a 

 minute, but it is usually of less frequent occurrence. 



