II PROTOPLASM — GEOGRAPHICAL DISTRIBUTION 47 



p. 150). Again, it may form transitory or more or less permanent 

 pseudopodia,^ (1) blunt or tapering and distinct, with a hyaline 

 outer layer, or (2) granular and pointed, radiating and more or 

 less permanent, or (3) branching and fusing where they meet into 

 networks or perforated membranes. Cilia or flagella are motile 

 thread-like processes of active protoplasm which perforate the 

 pellicle ; they may be combined into flattened platelets or firm 

 rods, or transformed into coarse bristles or fine motionless sense- 

 hairs. The skeletons of the Protozoa, foreign to the cytoplasm, 

 will be treated of under the several groups. 



Most of the fresh-water and brackish forms (and some marine 

 ones) possess one or more contractile vacuoles, often in relation 

 to a more or less complex system of spaces or canals in Flagel- 

 lates and Ciliates. 



The Geographical Distribution of Protozoa is remarkable for 

 the wide, nay cosmopolitan, distribution of the terrestrial and 

 fresh-water forms ; ^ they owe this to their power of forming 

 cysts, within which they resist drought, and can be disseminated 

 as " dust." Very few of them can multiply at a temperature 

 approaching freezing-point ; the Dinoflagellates can, however, and 

 therefore present Alpine and Arctic forms. The majority breed 

 most freely at summer temperatures ; and, occurring in small 

 pools, can thus achieve their full development in such as are 

 heated by the sun during the long Arctic day as readily as in 

 the Tropics. In infusions of decaying matter, the first to appear 

 are those that feed on bacteria, the essential organisms of 

 putrefaction. These, again, are quickly followed and preyed 

 upon by carnivorous species, which prefer liquids less highly 

 charged with organic matters, and do not appear until the 

 liquid, hitherto cloudy, has begun to clear. Thus we have 

 rather to do with "habitat" than with "geographical distri- 



^ Lang distinguishes "lobopodia," "filopodia," and "pseudopodia" according to 

 their form, — blunt, thread-like, or anastomosing. In some cases the protoplasm 

 shows a gliding motion as a whole without any distinct pseudopodium, as in Amoeba 

 Umax (Fig. 1, p. 5), and a pseudopodium may pass into a thin, active flagellum, 

 which is, however, glutinous and serves for the capture of prey : such often occurs 

 in the Lobosa Podostoma and Arcuothrix, which are possibly two names for one 

 species or at least one genus ; and in many cases a slender pseudopodium may be 

 waved freely. 



2 See Schewiakoff, " Ueb. d. Geograph. Verbreitung d. Siisswasserprotozoen," 

 in Mim. Acad. St. Paersb. ser. 7, xli. 1893, No. 8. His views apply to most 

 minute aquatic organisms— Animal, Vegetable, or Protistic. 



