PEOTOZOA AND SPONGES. 437 



Another genus somewhat similar is Arcella, but it 

 diflfers in being furnished with a more 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, we 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 fii-st examination 

 they seem inert and dead, but if we closely watch one, 

 we perceive that it is endowed 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 

 irregular motion is given to the box, which is some- 

 times turned partly over ; when we perceive that its 

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

 line is cut into facets. The lorica 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 through the coloured lorica, and resemble 

 those of Amaha. 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 Amo&ba, whose external surface has the power of 

 secreting a symmetrical shell of horny, or chitinous 

 substance. The lorica is about j^^th of an inch in 

 diameter. This species is named Arcella vulgaris. 



Laying aside our live-box with its contents for 

 the present, we will have recoui-se to the tank of 

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



