OF ANIMALCULES. 123 



new genera, called by Dr. Ehrenberg Arcella, which 

 I have thought would best be described after the TW- 

 choda sol. Dr. E. has observed three species, but has 

 only figured the Vulgaris, whose crustaceous covering is in 

 the shape of a cup, at the circumference of which radiate 

 several strong spines, as shewn at figure 155 ; and at 

 the edge-view, figure 154. 



The shell is delicately fluted, and of a pale brown co- 

 lour ; the body clear, like crystal. The spines, which 

 are attached to the body of the animalcule, and not to the 

 shell, can be contracted at pleasure. Sometimes it only 

 thrusts out two of them ; but when all are expanded, 

 seven are seen. The mouth aperture is on the flat side, 

 as shewn by the cross. Its polygastric structure may be 

 seen in the drawing. It varies considerably in size, 

 some full-grown specimens measuring in diameter 

 1 -240th, while others are only 1- 1200th of an inch. 

 Ehrenberg, in his description, states that he has, since 

 his systematic arrangement was printed, discovered an 

 allied genus, which he calls the proper Difflugia : it is 

 distinguished by a pointed projection from the surface of 

 the shell, and is larger than the Arcella : he names it 

 Difflugia acuminata, or the pointed changeable animal- 

 cule : like the other, it will not feed on colouring par- 

 ticles. 



237. Trichoda Solaris. — This animalcule is rather less 

 than the sun Trichoda; spherical, with seldom more 

 than seventeen wavy hairs on the circumference. In ad- 

 dition to the polygastric sacs seen within it, there may 



