The Sarcodina 245 



such reports have encountered scepticism, they suggest the desirabihty of 

 further investigation under conditions which would eliminate possible 

 contamination of cultures with other forms of Protozoa. 



Phenomena suggesting syngamy also have been reported in Testacida. 

 In the usual account, two mature organisms fuse with the mouths of 

 their tests in contact and the binucleate mass is drawn into one test (36, 

 42, 106, 142). Unfortunately there is no real evidence that meiosis and 

 the fusion of haploid nuclei occur. Until such data are available, inter- 

 pretations must remain tentative. However, the actual occurrence of such 

 cytoplasmic fusions (Fig. 5. 30, D-H) is attested not only by descriptions 

 of stained material but also by continuous observations on living speci- 

 mens (87). These findings, in conjunction with the occurrence of syngamy 

 in Heliozoida and Foraminiferida, stress the need for more intensive 

 study of life-cycles in Testacida. 



Although Testacida are generally capable of surviving drought — re- 

 maining viable for some time on dried moss, for example — they often do 

 not develop typical cysts. Instead, the pseudopodia are withdrawn, usually 

 bringing into the mouth of the test a mass of debris which forms a plug. 

 Inside the test, a chitinous membrane is secreted (Fig. 5. 30, C). The 

 result is an effectively sealed "capsule-stage" (151), seemingly quite re- 

 sistant to desiccation. Perhaps less commonly, true cysts (Fig. 5. 30, A, B) 

 are produced. In such cases, the reserve shell-plates of Euglyphidae may 

 be used for a "cyst-shell" within the test. A cyst membrane is then 

 secreted inside the cyst-shell. 



Ecological relationships. The Testacida as a group, and many of the 

 individual species, are cosmopolitan inhabitants of fresh water. Eco- 

 logically, however, their distribution is more restricted. Some of the 

 Testacida are commonly found in wooded areas or along streams on moss 

 which is not constantly submerged. Others are typical of fauna reported 

 for peat bogs, and a few species are commonly found in deep lake ^vaters 

 (depths of 60 feet or more). In general, the Testacida thrive best in acid 

 waters and may be either rare or absent in neutral or alkaline waters. 

 In surveys of various European bogs, species representing 18 genera and 

 all three families have been found within the range, pH 5.0-6.4. Within 

 these limits, differences in pH seemed to show little correlation with 

 specific composition of the fauna, but relatively few organisms were found 

 in an environment at pH 4.6 (103). The Testacida may prove interesting 

 material for studying the relations of pH to the utilization of minerals. 



Taxonomy. Subdivision of the order is usually based upon the structure 

 of the test. Genera with a secreted test, either apparently homogeneous 

 or containing minute structural elements, are assigned to the family 

 Arcellidae. The family Difflugiidae is characterized by arenaceous tests, 

 composed usually of sand grains although sometimes of other materials. 

 The test of the Euglyphidae shows an outer layer of scales, or plates. 



