176 



HARDWICKE'S SCIENCE-GOSSIP. 



^'S- 153- Side view of large specimen, whose test 

 is composed of large and small sand-grains. 



Fig. 154. Same test, ventral aspect. 



Figs. 15s and 156. Front and side aspects of a 

 dark brown variety, test, sand and dirt. 



F'g- 157' Test of smoky yellow chitine, incrusted, 

 posteriorly with flocculent dirt, but no sand-grains or 

 diatoms. From Sphagnum. 



The genus Arcella disputes with the previous form 

 the honour of being the commonest of the testaceous 

 Rhizopods. They are generally found in association, 

 and there are few microscopists who are not familiar 



Fig. 152. — Centropy.xis acnlcata. 



Fig- i5i-—Ct)tlrofiyxis aculeata. 

 (Side \ie\v.) 



Fig. \n.—Ccntropyxis aculcata. 

 (Ventral aspect.) 



with their brown tests. Arcella is sometimes very 

 numerous, not only in the light sediments of our 

 ponds, but also among the Algae clothing the sides. 

 It was discovered by the illustrious Danish natural- 

 ist Ehrenberg about sixty years ago. There are 

 many varieties, some very striking ones, but the 

 forms gradually merge into each other, so that many 

 authorities consider them as one species. Leidy 

 gives five specific names to the more widely separated 

 forms, two of which, A. communis and A. discoides, 

 are found in all the waters of this district. The 

 shell is composed of chitine, light or dark brown in 



colour, though occasionally I have found colourless 

 specimens. This membranous test has a minutely 

 hexagonal, cancellated structure, something like the 

 marine diatom Coccinodiscus, though this is not 

 always demonstrable. Many of the varieties have 

 really elegant shells, the dome of some having 

 angular facets, or pits, which being thinner and 

 lighter in colour than the connecting parts, have a 

 charming effect. In A. dentata there are a variable 

 number of recurved spines, arranged round the 

 circumference of the shell, and I figure a form in the 

 ventral aspect, somewhat approaching this, though 

 not sufficiently so to justify its being placed in that 

 species. 



In A. viilgaris or communis, as it is indiscrimately 



Fig. iss.— Ce';/>v/y.t2s aciilt'ntiX 



Fig. z5C:)- — CeHiroJiy.xis acuieata. 



Fig. 157. — From Sphagnum, above Billaden. 

 Test of smoky yellow Chitine incrusted with 

 dirt, but no sand-grains or diatoms. 



called, the test as generally seen is a brown circular 

 disc, with a central round opening for the emission 

 of the pseudopods. On a side view, the outline is a 

 low campanulate, with the basal border rounded, and 

 the mouth inverted. The height is about half the 

 breadth. 



The sarcode is colourless and the mass rests on the 

 base and around the inverted funnel of the mouth, 

 and is connected with the fundus, or top of the shell, 

 by threads of the ectosarc. There are two nuclei, 

 one on each side, and several cont. vesicles. These 

 details can only be made out in transparent speci- 



