CHAP. XIV The Simplest Animals 213 



ot cell-life, it is natural to find that the very simplest Protozoa, such 

 as Protomyxa, exhibit a cycle of amoeboid, encysted, and flagellate 

 phases, not having taken a decisive step along any one of the three 

 great paths. Moreover, the cells of higher animals may be classified 

 in the same way. The ciliated cells of the windpipe or the mobile 

 spermatozoa correspond to Infusorians ; mature ova, fat-cells, de- 

 generate muscle-cells, correspond to Gregarines, while white blood- 

 corpuscles and young ova are amoeboid. 



3- The common Amoeba. — To find Amoebae, which is not 

 always easy, some water and mud from a pond should be allowed 

 to settle in a glass vessel. Samples from the surface of the sediment 

 should then be removed in a glass tube or pipette, dropped on a 

 slide, and patiently examined under the microscope. Among the 

 debris, traversed in most cases by swift Infusorians, the sought -for 

 Amceba may be seen, as an irregular mass of living matter, often 

 obscured with various kinds of particles and minute Algas which it 

 has engulfed, but hardly mistakable as it ploughs its way leisurely 

 among the sediment, sending out blunt and changeful finger-like 

 processes in the direction towards which it moves, and drawing 

 in similar processes at the opposite side. From some objects it 

 recoils, while others of an edible sort it surrounds with its blunt 

 processes and gets outside of. Intense light makes it contract, and 

 a minute drop of some obnoxious reagent causes it to round itself off 

 and lie quiescent. Such is the simple animal which, in 1755, an 

 early microscopist Rosel von Rosenhof was delighted to describe, 

 caUing it the " Proteus animalcule." 



4- Structure of the Protozoa.— Most of these Protozoa are 

 units or single cells, but this contrast between them and the higher 

 animals is lessened by the fact that many Infusorians, some 

 Radiolarians, and some of the very lowest forms live inclose combina- 

 tion, a number of apparent individuals being substantially united in 

 co-operation. In two quite different ways this compound life of some 

 Protozoa arises. The ' ' Flower of Tan " {Fuligo or ^thaliuin 

 septicum) which in the summer months spreads as a yellowish slime 

 on the bark of the lanyard, and supplies the student with the 

 " largest available masses of undifferentiated protoplasm," arises from 

 the flowing together and fusion of a number of smaller amoeboid 

 units. But in some Infusorians and Radiolarians the colony 

 arises quite otherwise. Protozoa multiply by division ; each unit 

 splits into two which thenceforth live separate lives, and by and 

 by themselves divide. Suppose, however, that the unit divide 

 incompletely ; suppose that the daughter -units, distinct though 

 unsevered, redivide, and that the process is continued ; a " colonial " 

 Protozoon is the result. In this case the units do not flow together, 

 tliey were never separated. But the " wisdom " of some of these 



