ARCELLIDA. 117 
Test sub-orbicular or hemispherical, chitinoid, with 
sand-egrains 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) Eurenpera in Abh. K. Akad. Wiss. Berlin, 
1830 (1832), p. 40, and Infus. (1838), p. 182. 
larity Carrer in Ann. Nat. Hist. (2) XVIII (1856), 
p. 247, 
Diffugia (pars) Watch 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 body 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 amcebule, 
escape, develop tests of the same pattern as the parent, 
