THE RHIZOPODA 143 



in appearance, so that a male cannot be distinguished from a 

 female form. 



The formation of gametes is not preceded by encystment in 

 Pelomyxa. The zygote resulting from conjugation frequently 

 encysts, but this is simply a protective measure, and has nothing 

 to do with reproduction. Similarly an Amoeba may often be 

 seen to encyst itself and pass into a resting stage, but after a 

 time the cyst-wall is absorbed, the Amoeba emerges and re- 

 sumes its active existence. There is therefore no necessary 

 connection between encystment and reproduction. 



Amoebae and their allies, distinguished from other Protozoa 

 by the possession of pseudopodia, are classed together as 

 Rhizopoda. Amoeba is one of the most simple members of 

 the group, and may be taken as a type of it, but it must not 

 be supposed that all Rhizopoda have so simple a structure. 

 Some of the nearest allies of Amoeba having, like it, blunt 

 lobose pseudopodia, have the power of forming shells of 

 characteristic form for the protection of their bodies. Thus 

 Difflugia, a common fresh-water Rhizopod, protects itself by 

 a case formed of particles of sand glued together by a secretion 

 of the protoplasm. Arcella, another common fresh-water 

 form, has a watch-glass-shaped shell formed of a chitinoid 

 substance. The concavity of the watch-glass is covered in 

 by a plate with a central hole through which the pseudopodia 

 are protruded. Not far removed from these, and differing 

 from them chiefly in the fact that their pseudopodia are long 

 and thread-like, frequently branched at their extremities, and 

 anastomosing so as to form a protoplasmic network outside 

 the cell-body, are Euglypha, a fresh-water Rhizopod, with an 

 ovoid shell formed of hexagonal siliceous plates, and Micro- 

 gromia with a chitinous shell which is not made up of plates, 

 but is continuous. Closely allied to these, again, are the 

 Foraminifera, a very large group of rhizopodous Protozoa, 

 with calcareous shells, many of which are of great beauty and 

 complexity. The Foraminifera are all marine, and are ex- 

 tremely abundant. Some of them are pelagic that is to say, 

 they float at or near the surface in the open ocean far away 

 from land; and the bottoms of those oceans whose depth 

 does not exceed 2000 fathoms are generally covered with 

 deposits of a grey mud, which is chiefly composed of the 

 calcareous shells of the countless Foraminifera which have 



