PROTOZOA AND SPONGES. 883 



chitinous substance. The lorica is about — !— th of an inch 



500 



m diameter. This species is named Arcella vulgaris. 



Laying aside our live-box with its contents for the 

 present, we will have recourse to the tanks of sea-water 

 for one or two other objects of intermediate interest. On 

 the green and brown mossy sea-weed which covers the 

 rocks on the bottom, you see many white specks clinging 

 to the filaments ; and there are several adhering to the 

 sides of the tank. These are little living shelled animals 

 of the class Foraminifera, and these which you see include 

 several species. By bringing your eye, assisted by the 

 lens, to bear upon one of these latter, you perceive that it 

 is a little discoid spiral shell, of very elegant form, 

 marked with curved diverging grooves. This is the pretty ' 

 little Polystomella crispa, a fair sample of its class, and 

 though not more than -g^th of an inch in diameter, it is a 

 giant compared with the Arcella. 



There is more, however, than the shell to be seen ; 

 though so filmy and shadowy, that I wonder not at your 

 overlooking it. Extending from two opposite sides of the 

 shell to a distance each way considerably exceeding its 

 diameter, you discern fine threads of clear jelly, running 

 out in long points. The power you employ is not suffi- 

 cient to enable you to resolve their detail : and for this I 

 will try to secure a specimen for the microscope. 



In this other live-box, then, I inclose one of the white 

 specks from the moss-like clothing of the stones. It is, 

 I see, of another species, namely, polymorphina oblonga, 

 but it will answer our purpose equally well. 



At present we see only the shell, the removal of the 

 animal having induced it in alarm to withdraw the whole 

 of its softer parts within the protection of its castle. We 

 must have a few minutes' patience. 



Now look again. From the sides of the opaque shell 

 we see protruding tiny points of the clear sarcode ; these 

 gradually and slowly, — so gradually and slowly that the 



