Arcellina.'] infusoeial ANTMALcrLES. 211 



from the border of their h^rica. Pressure fractures their lorica like a 

 brittle substance. By the cracks so formed, the contained substance 

 escapes, extending iu the form of contractile expansions, as in the 

 Amoeba. I have seen one larger lobe almost separated, as if about 

 to become an independent being. M. Peltier has observed contact 

 to take place between the expansions of neighbonring Arcella without 

 any imion being effected, while the processes of the same Arcella 

 will unite and become blended together. 



The lorica in young Arcella is extremely diaphanous, and gra- 

 nulations or striae are to be seen only in those of lai-ger size, hence it 

 may happen with respect to some species that they represent but dif- 

 ferent stages of existence of the same animal." 



A. vulgaris. — Lorica, round and bell-shaped, with a hemispherical 

 or turgid back ; smooth, and composed of rows of minute granules ; 

 colour yellow or reddish brown. Found abundantly amongst lemna 

 and aquatic plants. Size 1-5 70th to 1 -240th. 



A. aculeata. — Lorica hemispherical, though often mis-shapen and 

 spinous at the margin. It is formed of short spicula, and is of a 

 yellowish colour. The spines sometimes issue from only one-half of 

 the margin of the shell, or shield-like lorica ; the shell is not readily 

 desti'oyed by heat. Fig. 92 represents one of these creatures with 

 the projecting spines, and the large round opening in the lorica. 

 Fig. 93 represents another creatui-e with three spines projecting from 

 its lorica, and a single variable process issuing from the under side ; 

 the digestive cells may also be seen. Fig. 94 shews an empty de- 

 formed lorica. Diameter of lorica 1-2 10th. 



A. dentata. — Lorica membranous and homogeneous ; of an hemis- 

 pherical or polygonal form, having the margin dentated ; colour 

 yellowish or greenish. Foimd amongst conferva. Size 1 -570th to 

 1 -240th. 



A. (?) hyalina. — Lorica membranous, smooth, and approaching to 

 globular, smaller than the preceding ; colourless. Foimd in matter 

 deposited by water, along with Cyphidium awreolum, &c. Size 

 1-1 150th to 1-5 70th. 



A. Americana. — oblong ; aperture small, round, not in the median 

 line. 



A. constricta. — Lorica ovate ; sHghtly contracted about the foramen, 

 \\'hieli is very large and to one side. 



