204 THE PROTOZOA 



the differences slowly disappear by the diffusion of the one liquid 

 into the other. 



Chemotaxis is a phenomenon which is obviously of the greatest 

 importance in the natural life of the organism. It comes into play in 

 the search for food and in sexual attraction, for example. It has long 

 been known that certain Protozoa are attracted towards food- 

 substances, especially those species which feed more or less 

 exclusively upon certain particular foods. Plasmodia of Mycetozoa, 

 for example, "scent" their food from a considerable distance, and 

 move towards it. 



Rhumbler (34, 204) has studied the ingestion of food by amoebae, and lias 

 made a number of experiments on the manner in which drops of fluid take up 

 or cast out solid particles. Thus, a drop of chloroform suspended in water 

 draws into its interior a glass splinter coated with shellac when brought into 

 contact with it ; after a time the coating of shellac is dissolved in the chloro- 

 form, and the glass splinter is then ejected from the drop. This experiment 

 furnishes data for a mechanical explanation of the ingestion of food and 

 ejection of faecal matter ; and it might be expected that amoebae in Nature 

 would ingest mechanically, and as it were helplessly, many substances of a 

 useless kind with which they are brought into contact. This may occur 

 experimentally when amcebse are brought into contact with substances of 

 no nutritive value ; Rhumbler observed an amoeba which ingested carmine- 

 particles until it died. In Nature, however, there can be no doubt that 

 amoebae exercise a certain choice or selection in the food they ingest, doubtless 

 as the effect of rhemotactic reactions (compare Jennings, 168). In the 

 Ciliata, however, tnere appears to be no selection of the food-particles wafted 

 down the oesophagus except as regards their size (compare Greenwood, 162). 

 Purely mechanical reactions, on the other hand, may possibly explain the 

 apparent selection which many Protozoa exhibit in building up houses of cer- 

 tain special materials (p. 34). 



Chemotactic reactions to particular substances must play a large part 

 in determining the migrations of certain parasitic Protozoa towards particular 

 organs of the body in which they are parasitic, in so far as such migrations 

 are not purely passive on the part of the parasite, or determined to some extent 

 by rheotaxis (see below). 



The attraction of gametes to one another can hardly be effected by any- 

 thing but chemotaxis. It is well known that the antherozoids of the fern- 

 prothallus are positively chemotactic to malic acid, which is secreted by the 

 oogonium. In Coccidium schubergi, Schauclinn (99) observed that the macro- 

 gamete, as soon as it had expelled its karyosome, but not before, became 

 attractive to the microgamete. 



The effects of drugs and reagents on the activities of the Protozoa is a 

 field of investigation which cannot be dealt with in detail here. Some 

 reagents have a quickening effect on the movements, others the contrary. 

 Narcotics, on the other hand, such as alcohol, ether, etc., may at first have 

 a stimulating, later a deleterious, action on the vital activity. Minute doses 

 of alcohol, according to Woodruff (216), diminish the rate of division at one 

 period, augment it at another, of the life-cycle, but in the latter case the rate 

 is not continuous, but decreases again ; increase in the amount of alcohol 

 will, however, again cause a more rapid cell-division for a limited period. 

 Thyroid extract is stated to have an attractive effect on Paramecium, and also 

 increases its capacity for reproduction (Nowikoff, 183). For the effects of 

 other drugs and poisons, see Giemsa and Prowazek (159), and Prowazek 

 (191, 192, and 195). In the same culture different individuals often exhibit 

 different powers of resistance to the effects of reagents. 



