340 BIOLOGY OF THE PROTOZOA 



A typical alternation of generations comparable with that of the 

 Foraminifera was described by Schaiidinn for the peculiar genus 

 TrichosphoBrium. Here asexual processes occur by irregular plasmic 

 divisions (plasmotomy) and by multiple division resulting in a 

 swarm of minute naked amoebfe. These develop into an adult form 

 of different type which may likewise undergo plasmotomy leading 

 to the formation of gamonts and gametes. The latter, upon fer- 

 tilization, give rise to the initial type of organism. In this cycle, 

 the original asexual generation differs from the later sexual genera- 

 tion by the presence of a peculiar type of test consisting of radially- 

 arranged spicules of magnesium carbonate. 



The forms included in this Order fall naturally into two families— 

 Arcellida^ and Euglyphidse (see key for genera). 



Family 1. Arcellidae.— Tests transparent or opaque by reason 

 of covering of foreign bodies picked up by the protoplasm and 

 deposited on the outside where they are cemented to the chitinous 

 membrane. 



Structure and materials of the shell afford a basis for further 

 classification of the family. They are either pyriform or shaped 

 like a watch glass; the membrane may be rigid or flexible and the 

 aperture central or asymmetrically placed. 



Sub-family ArceUince.— The tests are watch-glass shape, mem- 

 branous or chitinous in character. A definite aperture is wanting 

 in P.s-eudochhimys, the test resembling an inverted saucer over the 

 organism. A central aperture is present in the other genera except 

 in Centropyxis where it is asymmetrically placed. It occupies most 

 of the ventral surface in Pyxidicula, but is relatively small in others 

 and variously modified: (1) Arcella with margin of the test inturned 

 at the oral opening; (2) without the inturned collar in Capsellina; 

 with a velum or distensible membrane about the aperture as in 

 Microcorycia; (.3) with aperture slit-like instead of circular, as in 

 ParmnUna. In one genus: DipJochUmiys the membrane is double, 

 the outer bearing foreign ]:»articles. 



Sub-family Dlffliigii nee. —Here the tests are pyriform, not com- 

 pressed, and covered with foreign particles of diverse kinds. They 

 may be arcelliform but asymmetrical and with an arcuate aperture 

 as in BuUinuIa; or spheroidal with a narrow slit-like mouth as in 

 Plagiojjy.ra; or even spiral, with a distinct neck as in Lesquerrusia. 

 The majority of tests are symmetrical, in some forms with a long 

 neck as in CneurhiteUa; in others an internal diaphragm forms a 

 neck-like constriction as Pontigulasia. Large pyriform types with 

 different kinds of foreign bodies, and opaque, are included in the 

 genus Difflugia, while smaller forms of similar shape, but somewhat 

 compressed and transparent, are placed in the genus Cryptodifflugia. 

 Pseudopodia are generally of the filopodia type but in DifflugirUa, 

 both lobopodia and filopodia are present, while in Phryganella, the 



