454 BIOLOGY OF THE PROTOZOA 



limited to the region adjacent to the orifice of the shell. In some 

 cases, as in the genus Cochliopodium, there is a firm ectoplasm which 

 has many of the features of a chitinous membrane. Pseudopodia 

 pass through it by way of permanent apertures (Fig. 9, p. 31), 

 and when the cell divides the membrane also divides. There arc 

 very few of such forms, however, the great majority of shelled forms 

 having a definite chitinous membrane on which foreign particles 

 are attached. In Arcellidae the membrane is clear chitin and in the 

 Euglyphidae the outer elements of the shell are secreted before divi- 

 sion and passed out to the daughter individual after the chitin 

 membrane is laid down. The variety of shells is due to the different 

 types of sand crystals, diatoms, detritus of various kinds and even 

 living plant cells. 



The nucleus is vesicular and usually single although many types 

 of both naked and shelled forms are binucleated or multinucleated. 

 The entire group is further characterized by the distribution in the 

 cytoplasm of chromidia (see p. 09) which often takes the form 

 of a chromidial network. 



With the exception of the parasitic forms, and some of these arc 

 also included, the Amoebaea are holozoic in nutrition and proteo- 

 lytic and amylolytic ferments have been isolated in some cases (see 

 Chapter V). 



Notwithstanding the abundance and the wide distribution of 

 these forms of rhizopods there is very little agreement on the part 

 of different observers in regard to the life history. Few Protozoa 

 have been more frequently seen and studied than Amoeba proteus 

 and yet little is known accurately about the life cycle. Binary 

 division is characteristic of all the naked forms both free-living and 

 parasitic, and encystment stages are known in all forms. So-called 

 budding division is typical of the testate forms and differs materially 

 from binary fission (see p. 214). Acceptable accounts of sexual 

 processes are limited to the Testacea in which there is a general 

 resemblance to the type of gamete formation characteristic of the 

 Foraminifera (see Chapter VI). 



Parasitic forms of the Amoebidae are widely distributed through- 

 out the animal kingdom. They are usually present in the diges- 

 tive tract but may be ectoparasites as well. The great majority 

 are of the nature of commensals and are harmless, some, however, 

 are pathogenic as Amoeba mucicola Chatton, a harmful ectoparasite 

 on the gills of Labridae, or Endamoeba dysenteriae, the cause of 

 dysentery in man (see p. 387). 



The organisms included in the Amoebaea fall naturally in one of 

 two groups which have been generally recognized as Amoebida 

 (Gymnamoebida) and Testacea. Following the principle adopted 

 in classifying the Mastigophora where ameboid forms of animal 

 flagellates are retained as Mastigophora only when the flagellum or 



