PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 73 



day, for biologists recognize that it is only an academic matter after 

 all, and merely affords further evidence of the artificiality of classi- 

 fication. 



It is to these intermediate forms that we must turn for the causes 

 of odors and tastes, which occasionally make potable waters unfit to 

 drink. As shown in the previous chapter, the metaplastic products of 

 vital activity are sometimes stored up in the cell as oils or fats, which, 

 when liberated in a water supply, give rise to offensive odors and tastes 

 (Fig. 2). Like all organisms which make their food, these sus- 

 pended protozoa require salts of different kinds. Many such salts are 

 normal to drinking waters, the nitrites and nitrates being almost 

 invariably present, and these are the very salts most needed for the 

 maintenance of these forms of life. Hence, it follows that if an 

 infected water supply can be freed from an excess of such nitrogen- 

 holding salts, the protozoa will disappear. If inlet and outlet of a 

 given water supply are closed, the organisms soon exhaust the avail- 

 able food elements and die. 



While some forms of protozoa are thus holophytic, like the green 

 plants, others combine the holophytic with the animal, or holozoic 

 method, while still other protozoa, and, indeed, the great majority of 

 them, are entirelv holozoic. Thev seize their food in the form of other 



tl t, 



minute living things and digest it in much the same way that higher 

 animals do, all of the organs of the cell playing some part in the pro- 

 cess.. Food-getting, therefore, more than any other function of the 

 body, has been the most influential in leading to morphological 

 development. 



Seizure of food is one of the most interesting of the protozoon pro- 

 cesses, and is frequently accompanied by such complicated reactions 

 on the part of the minute animal as to suggest wilful activity. In 

 other cases it is quite mechanical, as, for example, in choanoflagellates, 

 or in many ciliates. In these the motile organs, flagella, or cilia, 

 create a current in the surrounding water toward the mouth, and this 

 carries with it bacteria or minute pieces of disintegrated plant or animal 

 matter. In Vorticella campanula and its allies the apparatus is most 

 highly developed for this method of food taking. A powerful adoral 

 zone of membranelles creates a vortex current toward the oral or 

 vestibular opening, while within the vestibule a long, undulating 

 membrane carries the current to the mouth opening. The proto- 

 plasmic area around the mouth is furnished with contractile muscle 

 threads or myonemes, so that when any irritating object comes with 

 the food current, the entire vestibular area, adoral zone and all, con- 

 tracts into the cell body, while the myonemes of the distended stalk 

 contract at the same time and draw the body away from the offending 

 region. In other ciliates, like paramecium, colpidium, oxytricha, 

 etc., the process is essentially the same except that the animal is not 



