ARCELLIDA. 117 



Test sub-orbicular or hemispherical, chitinoid, with 

 sand-grains or other extraneous matter adhering, the 

 pseudopodia not distinctly digitate, often elongated 

 and branched; more or less pointed. 28. Phryganella. 



Genus 15. ARCELLA Ehrenberg, 1832. 



Arcella (pars) EHRENBERG in Abh. K. Akad. Wiss. Berlin, 

 1830 (1832), p. 40, and Infus. (1838), p. 132. 



Arcellina CARTER in Ann. Nat. Hist. (2) XVIII (1856), 

 p. 247. 



Diffluyia (pars) WALLICH in Ann. Nat. Hist. (3) XIII 

 (1864), p. 245. 



Test membranous, composed of transparent chitinoid 

 material, turning brown with age, and densely punc- 

 tated. Form variable; in dorsal or ventral aspect 

 circular, with a central aperture ; in lateral view 

 varying from plano-convex to hemispherical, the mouth 

 turned inwards and appearing as an inverted funnel. 

 Externally the test is plain or (in certain varieties) 

 variously ornamented, having the dome moulded into 

 angular facets, pitted, or furnished with acute pro- 

 minences springing from an upturned border, such 

 prominences occasionally developing into spines. The 

 protoplasmic body is centrally situated within the test, 

 and attached to its internal margin, at various points, 

 by threads of ectoplasm. Pseudopodia few, digitate, 

 blunt, simple or branched ; nuclei (usually two) situated 

 on opposite sides of the body ; contractile vacuoles 

 (four or more) small, near the periphery. 



Reproduction in this genus is most commonly effected 

 by the formation of spores and by " budding," the 

 latter being regarded as an intermediate process 

 between simple fission and the complete breaking up 

 of the parent bocly into spores. Nine or more globular 

 processes have been observed (by Biitschli amongst 

 others) surrounding the mouth ; these get " pinched 

 off," and, after passing a short existence as amoebulae, 

 escape, develop tests of the same pattern as the parent, 



