26 BIOLOGY OF THE PROTOZOA 



not apply equally to the deep sea types. The littoral fauna of salt 

 water, like the fresh water forms, appears to have a cosmopolitan 

 distribution according to the observations of Gourret and Roesser 

 (1886), of Levander and of Hamburger and Buddenbrock in Europe, 

 while in North America the brackish waters are particularly rich 

 in number and variety of Protozoa. The pelagic and deep sea forms 

 appear to be unequally distributed; some types are apparently 

 limited to the Indian Ocean; others to the Atlantic, while many 

 tropical genera and species, especially of Radiolaria and Foramini- 

 fera, are not found in the polar seas and vice versa. Some strictly 

 pelagic forms, on the other hand, notably Tintinnidae, are found 

 on or near the surface of sea water in all parts of the world. 



Observations are sufficiently numerous to show that not only is 

 there a certain climatic distribution of salt water forms, but a ver- 

 tical distribution as well. Certain genera and species of Radiolaria 

 and Foraminifera are present in the surface waters but are rarely 

 found at the depth of from 600 to 3000 feet, while some families, 

 notably the Challengeridae and Tuscaroidae, are present only in 

 the extreme depths of the sea. 



Many species are sufficiently adaptable to live either in fresh, 

 brackish or salt water; indeed most of the common forms of rhizo- 

 pods, flagellates and ciliates seem to be equally at home in either. 

 Many types, however, sometimes entire groups of Protozoa, are 

 not so ubiquitous; the sub-class Radiolaria for example, comprising 

 more species than any other entire class of Protozoa, is exclusively 

 marine, while another large sub-class of the Sarcodina, the Fora- 

 minifera, comprises only a few fresh water representative species. 

 Many more types of Choanoflagellates are present in salt than 

 in fresh water. Ciliates are poorly represented in the deep sea, 

 although one family— Tintinnidae— is wonderfully rich in salt water 

 forms while fresh water forms are uncommon. Heliozoa, another 

 sub-class of the Sarcodina, on the other hand, are typically fresh 

 water forms with relatively few salt water representatives. 



The distribution of parasitic forms belonging to all groups of the 

 Protozoa obviously follows the distribution of their hosts, and we 

 know too little on this subject to generalize; where animals are 

 segregated the opportunities for parasitism are enhanced while 

 some climatic conditions are more advantageous than others for 

 the spreading of germs. Thus the blood-dwelling parasites are 

 more common in the tropics than elsewhere, the biological condi- 

 tions favorable to the intermediate transmitting hosts being largely 

 responsible for their numbers and variety. 



SIZE, FORM AND APPEARANCE OF PROTOZOA. 



Although Protozoa belong unquestionably to the microscopic 

 world their sizes vary within wide limits. Some are large enough 



