74 PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 



attached nor provided with contractile fibrils. In all of these ciliated 

 forms there is a definite and frequently very complicated mouth 

 opening, but in the flagellated forms, as a rule, there is no permanent 

 mouth, the entire anterior end of the cell forming a receptive area 

 for food products swept toward it in the current created by the flagel- 

 lum. This is a vortex current caused by the undulations of the long 

 flagellum, which, at the same time, moves in such a way as to describe 

 a cone whose apex is at the base of the flagellum and base at the tip. 

 In some cases, as in the collared flagellates or choanoflagellata, the 

 flagellum moves inside a protoplasmic, collar-like membrane, which, 

 like a pseudopodium, can be thrown out or retracted by the animal. 

 The surfaces of this collar are sticky, and small particles move down 

 it to the floor of the collar pit, where they are taken into the body. 



As the flagella and cilia are in constant action, and as the mouth is 

 always open for more, these protozoa become, as Ma upas pointed 

 out, the gluttons, par excellence, of the animal kingdom, while the oral 

 apparatus becomes strikingly modified and diversified. 



Not all protozoa, however, are so persistent in food taking, and 

 many of them, while provided with a mouth opening, keep the mouth 

 shut until a food particle is to be eaten. Such forms live upon larger 

 things than bacteria, and with them eating involves a regular swallow- 

 ing process. In some cases this is combined with the food-getting 

 activity of the flagella or cilia, and large particles of solid proteid 

 matter, either in the form of small organisms or of disintegrated 

 fragments of plant or animal brought with the current, are seized by 

 protoplasmic processes, as in Oikomonas termo, or the mouth opens 

 to swallow them, as in Didinium nasuium. There seems to be 

 a remarkable power of distention in these mouth openings, for a 

 didinium can take in an organism quite as large as itself (Fig. 21). 



In those forms of protozoa belonging to the group suctoria there is 

 no mouth opening, nor flagella or cilia to create food currents, but the 

 animals are provided with tentacles, often twice as long as the diameter 

 of the body, with which they seize passing organisms. Once seized, 

 the victim struggles for a short time and then becomes quiet, as though 

 paralyzed. Its protoplasmic contents are then sucked into the body 

 of the captor, or, in some forms, the protoplasm of the captor passes 

 into the body of the victim and there digests its meal. 



Many protozoa set a trap for their victims, so that they become 

 entangled as in a spider's web. This is the case with the majority of 

 the great group of rhizopods, especially the foraminifera and radio- 

 laria, where the pseudopodia form a network of branching protoplasm, 

 or a forest of protoplasmic spines, in which the streaming of granules 

 is constant, passing from the inner protoplasm of the shell outward 

 to the farthest tip of the pseudopodia. The sticky character of the 

 pseudopodia makes it difficult for any small animal to break away, 



