24 



BIOLOGY OF THE PROTOZOA 



to 13 species and represent the same types for the most part as those 

 listed by Woodruff. Protozoa are very apt to stick to solid sub- 

 stances when they encyst and are carried, in the dried state, with 

 such substances, which accounts in part for the appearance of 

 Protozoa in all kinds of infusions. Similar adhering cysts may be 

 carried from place to place by birds and other flying creatures or 

 by land animals, thus helping to maintain a common type of proto- 

 zoan fauna in pools and casual waters. The commonest species of 

 Paramecium, viz., P. aurelia and P. caudatum, are widely distrib- 

 uted over the earth and are almost universally used in general 

 laboratory work as examples of ciliated Protozoa. Their mode of 

 distribution, however, has been a continued puzzle for their sup- 

 posed inability to form cysts has been generally recognized. Re- 

 cently, however, Cleveland (1927), upon injecting unknown species 

 of Paramecium in the rectum of a frog, found that a definite cyst 

 membrane is formed bv manv of the Paramecia. After a few 



Fig. 5. — Paramecium caudatum, stages in encystment. The final product may 

 be easily mistaken for a sand grain. (After Michelson, Arch. f. Protistenkunde, 

 courtesy of G. Fischer.) . 



days division within the cyst and ex-cystation were observed. 

 Michelson (1928), furthermore, has described encystment of Para- 

 mecium caudatum under conditions of slow desiccation entailing loss 

 of peristome, vacuoles and cilia. When fully dried the crumpled 

 cyst wall resembles a small sand grain and as such may be over- 

 looked (Fig. 5). 



Some forms to which Lauterborn (1901) has applied the term 

 "sapropelic fauna" appear to be able to live without free oxygen. 

 Thus Frontonia leucas, Prorodon ovum, Spirostomum ambiguum, 

 Pehmyxa palustris, P. binucleata, etc., which usually live in rela- 

 tively clear waters, may also live in the sulphurous medium of 

 putrefying vegetable and animal matter, while certain species of 

 ciliates of fantastic form seem to require this peculiar habitat for 

 their vital activities (Dactylochlamys pisciformis, Lauterb., Saprodi- 

 nium dentatum, Lauterb., Discomorpha pectinata, Levand., Pelodt- 

 nium reniforme, Lauterb.). Doflein, following the suggestion made 

 earlier by Bunge, believed that the anaerobic parasitic forms of the 



