PROTOZOA AND SPONGES. 437 



Another genus somewhat similar is Arcella, but it 

 differs in being furnished with a iriore or less rounded 

 shell {lorica), like a little box. In examining the mat- 

 ters that adhere to the stems of Duckweed, and other 

 water plants, Ave frequently observe little circular 

 bodies of a yellowish or reddish brown colour, some 

 much darker than others, but all having a central 

 round spot paler than the rest. On first examination 

 the}^ seem inert and dead, but if we closely watch one, 

 we perceive that it is endo"wed with the power of mo- 

 tion ; and we directly discern thrust out from its edge, 

 variable processes, in the form of arms, of clear, per- 

 fectly colourless, and most delicate jelly, sometimes 

 pointed, sometimes blunt, which slowly change their 

 form and position. By the aid of these, a feeble and 

 ii-resfnlar motion is 2;iven to the box, Avhich is some- 

 ti/nes turned partly over; when we perceive that its 

 under-side is fiat or probably concave, and that its out- 

 line is cut into facets. The loi^ica is somewhat flexible, 

 for the edges at two opposite points are sometimes bent 

 down towards each other, so as to give the creature the 

 form of a crescent. The internal viscera are dimly 

 discernible throufjh the coloured lorica, and resemble 

 those oi' Amosha. A dark oval ring is commonly seen 

 at one side, which is probably the outline of the 

 contractile bladder. It may, in fact, be considered as 

 an A7ncel>a, whose external surface has the power of 

 secreting a symmetrical sliell of hornj^, or chitinous 

 substance. The lorica is about j\-ot^\ of an inch in 

 diameter. This species is named Arcella vulgaris. 



Lavinjr aside our live-box with its contents for 

 the present, we will have recourse to the tank of 

 sea-water for one or two other objects of interme- 



