PROTOZOA 



It is not in detached fragments of the tissues of the higher 

 animals that we can best carry on this study : for here the cells 

 are in singularly close connexion with their neighbours during 

 life ; the proper appointed work of each is intimately related to 

 that of the others ; and this co-operation has so trained and 

 specially modified each cell that the artificial severance and 

 isolation is detrimental to its well-being, if not necessarily fatal 

 to its very life. Again, in plants the presence of a cell-wall 

 interferes in many ways with the free behaviour of the cell. But 

 in the blood and lymph of higher animals there float isolated 

 cells, the white corpuscles or " leucocytes " of human histology, 

 which, despite their minuteness (1/3000 in. in diameter), are in 

 many respects suitable objects. Further, in our waters, fresh or 

 salt, we may find similar free-living individual cells, in many 

 respects resembling the leucocytes, but even better suited for our 

 study. For, in the first place, we can far more readily reproduce 

 under the microscope the normal conditions of their life ; and, 

 moreover, these free organisms are often many times larger than 

 the leucocyte. Such free organisms are individual Protozoa, and 

 are called by the general term " Amoebae." A large Amoeba may 

 measure in its most contracted state 1/100 in. or 250 \x in 

 diameter, 1 and some closely allied species {Pelomyxa, see p. 52) 

 even twelve times this amount. If we place an Amoeba or a 

 leucocyte under the microscope (Fig. 1), we shall find that its 

 form, at first spherical, soon begins to alter. To confine our 

 attention to the external changes, we note that the outline, from 

 circular, soon becomes " island-shaped " by the outgrowth of a 

 promontory here, the indenting of a bay there. The promontory 

 may enlarge into a peninsula, and thus grow until it becomes a 

 new mainland, while the old mainland dwindles into a mere pro- 

 montory, and is finally lost. In this way a crawling motion is 

 effected. 2 The promontories are called " pseudopodia " ( = " false- 



liminary survey to designate lowly forms of life, not formed of the aggregation of 

 differentiated cells, we shall employ the useful term "Protista," introduced by 

 Haeckel to designate such beings at large, without reference to this difficult problem 

 of separation into animals and plants (see also p. 35 f.). 



1 The "micron," represented by the Greek letter /x, is 1/1000 mm., very nearly 

 1/25,000- of an inch, and is the unit of length commonly adopted for microscopic 

 measurements. 



' 2 A solid substratum is required, to which the lower surface adheres slightly : that 

 movement is complicated by a sort of rolling over of the upper surface, constantly 



