336 BIOLOGY OF THE PROTOZOA 



popular position of the lowest forms of animal life and the phrase 

 "from Amoeba to man" is familiar to everyone. They are present 

 in all stagnant fresh and brackish water; in damp moss or leaves; 

 abundant in the superficial soil, and also abundant as commensals 

 or parasites in all kinds of animals. 



In all of the naked forms there is a well-marked differentiation of 

 the protoplasm into endoplasm and ectoplasm. The latter is more 

 dense, the former more fluid and with typical cyclosis. In the 

 shelled typc^ there is frequently a characteristic zonal diflferentiation. 



Pseudopodia are never myxopodia or axopodia. Naked forms 

 have blunt finger-form processes or lobopodia formed by an outflow 

 of ectoplasm and endoplasm. Shelled forms in the majority of 

 t,^T3es, have pseudopodia composed apparently of ectoplasm only. 

 These have considerable power of movement apart from the usual 

 amoeboid type of flowing substance, and may sway or move inde- 

 pendently with A'igor. In the naked forms pseudopodia may be 

 thrown out from any part of the cell, but in shelled t\'pes they are 

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

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

 has many of the features of a chitinous membrane. Pseudopodia 

 pass through it by means of permanent apertures (Fig. S, p. 30). 

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

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

 having a definite chitinous membrane on which foreign particles 

 are attached. In Arcellid^e the membrane is clear chitin and in the 

 Eugl;\'phid8e 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 variet\- of shells is due to the different 

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

 living plants 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 granules of chromatin which originate from the 

 nucleus. 



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

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

 lytic and amylol\tic ferments have been isolated in some cases (see 

 Chapter IV). 



Notwithstanding the abundance and the wide distribution of 

 these forms of rhizopods there is ver\' little agreement on the part 

 of dift'erent observers in regard to the life history. Few Protozoa 

 have been more frequently seen and studied than Amoeba protevs 

 and yet nothing 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 



