SARCODINA RHIZOPODA 53 



it is a fresh-water dweller, living in the bottom ooze of ponds, 

 etc., richly charged with organic debris. It is remarkable also 

 for containing symbiotic bacteria, and brilliant vesicles with a 

 distinct membranous wall, containing a solution of glycogen. 1 

 Few, if any, of the Filosa are recorded as plurinuclear. 



The simplest Lobosa have no investment, nor indeed any 

 distinction of front or back. In some forms of Amoeba, how- 

 ever, the hinder part is more adhesive, and may assume the form 

 of a sucker-like disc, or be drawn into a tuft of short filaments 

 or villi, to which particles adhere. Other species of Lobosa and 

 all Filosa have a " test," or " theca," i.e. an investment distinct 

 from the outermost layer of the cell-body. The simplest cases 

 are those of Amphizonella, Dinamoeba, and Trichosphaerium, 

 where this is gelatinous, and in the two former allows the passage 

 of food particles through it into the body by mere sinking in, 

 like the protoplasm itself, closing again without a trace of per- 

 foration over the rupture. In Trichosphaerium (Fig. 9) the test 

 is perforated by numerous pores of constant position for the 

 passage of the pseudopodia, closing when these are retracted ; 

 and in the " A " form of the species (see below) it is studded with 

 radial spicules of magnesium carbonate. Elsewhere the test is 

 more consistent and possesses at least one aperture for the 

 emission of pseudopodia and the reception of food ; to avoid con- 

 fusion we call this opening not the mouth but the " pylome " : some 

 Filosa have two symmetrically placed pylomes. When the test is 

 a mere pellicle, it may be recognised by the limitation of the 

 pseudopodia to the one pylomic area. But the shell is often hard. 

 In Arcella (Fig. 10, C), a form common among Bog-mosses and 

 Confervas, it is chitinous and shagreened, circular, with a shelf 

 running in like that of a diving-bell around the pylome : there 

 are two or more contractile vacuoles, and at least two nuclei. Like 

 some other genera, it has the power of secreting carbonic acid 

 gas in the form of minute bubbles in its cytoplasm, so as to 

 enable it to float up to the surface of the water. The chitinous 

 test shows minute hexagonal sculpturing, the expression of ver- 

 tical partitions reaching from the inner to the outer layer. 



Several genera have tests of siliceous or chitinous plates, 



1 Stolfi in Z. wiss. Zool. lxviii. 1900, p. 625. Lilian Veley, however, gives reasons 

 for regarding them as of proteid composition, J. Linn. Soc. (Zool. ) xxix. 1905, p. 374 f. 

 They disappear when the Pelomyxa is starved or supplied with only proteid food. 



