CHAPTER XVIII 



ORDER 5 TESTACEA SCHULTZE 



/■T-VHis ORDER, which is also known as the Thecamoeba, com- 

 X prises those amoeboid organisms which are covered by a 

 single-chambered shell, or test, within which the body can be 

 completely withdrawn. The test has usually a single aperture 

 through which the cytoplasm is extruded as pseudopodia. The 

 test varies somewhat in shape and structure. A chitinous or 

 pseudochitinous membrane forms the basis of all tests. It may 

 be thickened, as in Arcella, or composed of foreign bodies 

 cemented together, as in Difflugia. In Euglypha silicious 

 platelets are formed in the endoplasm and deposited on the 

 membrane. 



The cytoplasm is well differentiated into the ectoplasm and 

 the endoplasm. The ectoplasm is conspicuously observable at 

 the aperture of the shell where the pseudopodia are formed. The 

 pseudopodia are slender and composed of ectoplasm only. Thus 

 they are mainly filopodia. The endoplasm is, on the whole, 

 granulated or vacuolated and contains food vacuoles, con- 

 tractile vacuoles and nuclei. The number of nuclei present in 

 a single individual varies from one to many. In some forms 

 there are present regularly in the cytoplasm numerous deeply 

 staining chromatin granules known as chromidia (p. 22). 



Asexual reproduction is either by longitudinal fission, in 

 forms with a soft shell, or by transverse division or budding. 

 In some forms multiple division occurs. Sexual reproduction by 

 amoeboid or flagellate gametes has been observed in a number 

 of species. Encystment is known to occur in the majority of 

 forms here grouped. The Testacea are mostly inhabitants of 

 fresh water, but a few live in salt water. A number of species 

 are semi-terrestrial, being found in moss or moist soil, es- 

 pecially peaty soil. 



This group is divided into the following four families: 



[225] 



