90 THE LOB OS A 



Schultze, is a common and widely distributed genus, with a shell of vari- 

 able shape, but distinguished by its regular pavement -like arrangement 

 of square or oblong plates (Fig. 20, B). Nebela, Leidy, is related to 

 Quadrula, but the plates of the shell are round, oval, or even irregular in 

 outline. In some species the shell is strengthened by adherent diatom 

 shells. In all species of this genus particles of "fat" of a pale blue or 

 yellow colour occur normally in the protoplasm. Similar particles also 

 occur in Difflugia and other genera, but are not so constant or characteristic 

 as they are in Nebela. 



The shell of Heleopera, Leidy, is provided with square or oblong 

 plates as in Quadrula, but they are usually irregularly or untidily 

 arranged. The pseudopodia of this genus are more numerous than in 

 the others of the family, and are sometimes slightly branched. 



In Phryganella, Penard, the shell is covered with adventitious particles, 

 as in Difflugia, but the pseudopodia are more numerous, more delicate, 

 frequently branched, and occasionally amalgamated at the base to form a- 

 membranous web. It appears to be related to Pseudodifflugia, Schlum- 

 berger, which is usually regarded as a member of the Order Gromiidea r 

 of the Foraminifera. As it is quite impossible to draw a definite line of 

 distinction between organisms with a few fine blunt pseudopodia such as 

 are characteristic of the Difflugiidae and those with filamentous branching 

 pseudopodia such as are characteristic of the Gromiidea, there is a group 

 of genera occupying an intermediate position between the Rhizopoda and 

 the Foraminifera. 



The principal genera of this group are : 



Cryptodifflugia, Penard ; Pseudodifflugia, 1 Schlumb. ; Diaphwodon,^ 

 Archer ; Platoum, 1 F. E. Schultze ; Clypeolina, Penard ; Nadinella, Penard; 

 Frenzelina, Penard ; Campascus, 1 Leidy ; Cyphoderia, 1 Schlumb. 



Family ARCELLIDAE. Shells plano-convex in shape, marked by a 

 very fine hexagonal pattern, not supported 

 by adventitious particles. 



Arcella, Ehr. This is a common and 

 widely distributed gemis. The shells of 

 the common species A. vulgaris vary from 

 80-140 p. in diameter, and like those v of 

 most of the species of Arcella are charac- 

 terised by their brown colour. The flattened 

 side of the shell is usually depressed and 

 perforated at the centre by the pylome, 

 FIO. 21. which is less than one-third the diameter 



Arcella vulgaris, Ehr. a, shell ; of the shell. From the pylome there 

 b, protoplasm within the shell ; c, . ,-, f , ' j , , 



lobose pseudopodia ; e, one of the project three or four, rarely more, digitate 



marginal vacuoles ; d, d, nuclei sur- pseudopodia. Situated in the ectoplasm, 



rounded by a halo of clear proto- x . . . , . 



plasm. (After Lankester.) and usually arranged in a circle round the 



pylome, there is often seen a series of 



vacuoles, which probably serve a hydrostatic function. They may fuse 

 together to form a single large excentric vacuole, and this may collapse 

 after the manner of a contractile vacuole. 



1 Cf. Treatise on Zoology, Part I. Fasc. II. pp. 140-141. 



