THE SARCODINA 229 



The swarm -spores are liberated from the cyst and live freely, feeding and 

 multiplying by binary fission, in which the nucleus divides by mitosis and the 

 Nebenkern acts like a centrosome. After a time, however, the swarm-spores 

 lose their flagella, and become amrebulae which develop into the adult phase. 

 Syngamy was not observed, but probably takes place between the flagellulae. 

 Two new parasitic species of Paramoeba have been described recently by 

 Janicki (71 "5) ; see p. 95. 



To the order Amoebaea should be referred, probably, the parasite of the 

 Malpighian tubules of the rat-flea (Ceratophyllus fasciatus), described by 

 Minchin under the name Malpighiella refringens, and the parasite of Ptychodem 

 minuta, described by Sun under the name Protoentospora ptychoderce. 



The section Lobosa Testacea or Thecamcebae contains a number 

 of free-living forms familiar to every microscopist, such as the 

 genera Difflugia (Fig. 16), Centropyxis, Arcella (Fig. 32), etc. The 

 majority of these forms inhabit fresh water, but Trichosphcerium 

 (Fig. 81) is marine. Their common distinctive feature, in addition 

 to the possession of lobose pseudopodia, is the formation of a shell 

 or house into which they can be withdrawn entirely. The shell may 

 be secreted by the animal, and then is chitinous (Arcella) or gelat- 

 inous (Trichosphcerium), or may be made up of various foreign 

 bodies cemented together (Difflugia). Typically the house has the 

 form of a chamber with a single large opening, through which the 

 pseudopodia are extruded at one pole. 



When the animal multiplies by fission, the protoplasm streams 

 out through the aperture, and forms a daughter-shell external to 

 the old one, after which division of the nucleus takes place and the 

 two sister-individuals separate. In Trichosphcerium, however, the 

 house has the form of a gelatinous investment to the body, with 

 several apertures through which pseudopodia protrude, and when 

 the animal divides the investing envelope divides with it. 



The protoplasmic body contains typically one nucleus some- 

 times more than one surrounded by a ring of chromidia. In 

 Arcella vulgaris there are constantly two primary nuclei ; in 

 Difflugia urceolata, from ten to thirty. Trichosphcerium possesses 

 many nuclei, but no chromidia. 



The life-cycle, so far as is known, is of various types ; those of 

 Arcella and Trichosphcerium are described above (p. 177, Fig. 80, 

 and p. 181, Fig. 81). The latter, with an alternation of generations 

 combined with dimorphism in the adult condition, approaches that 

 of the Foraminifera in character. 



In the testaceous amcebse the method of division varies in accordance 

 with the nature of the shell. In those in which the shell is soft and yielding, 

 as, for example, Cochliopodium and Cryptodifflugia, the division is longitudinal 

 i.e., in a plane which includes the axis passing through the mouth and 

 apex of the shell (Doflein, 239) ; in Cryptodifflugia rapid division of this kind 

 may lead to colony-formation. In forms with a rigid shell, on the other hand, 

 such as Difflugia, Arcella, Centropyxis, etc., the shell sets a limit to the growth 

 of the animal, which, when it has filled the shell, ceases to grow for a while 



