402 



EVENINGS AT THE MICROSCOPE 



sea- water in the compressorium. These threads are called 

 acontia. 



Examining this fragment under a low power of the mi- 

 croscope, we readily see that, though at first it seems a 

 solid cylinder, it is really a flat narrow ribbon with the 

 edges curved in, which can at pleasure be brought into 

 contact, and thus constitute a tube. Like all other in- 

 ternal organs in these animals, its surface is richly ciliated, 

 and the ciliary currents not only hurl along whatever float- 



PORTION OP ACONTIUM (flattened). 



ing atoms chance to approach the surface, but cause the 

 detached fragments themselves to wheel round and round, 

 and to swim away through the water. Though there is 

 not the slightest trace of fibre in the structure of the 

 acontium, when scrutinized even with a power of eight 

 hundred diameters, the clear jelly, or sarcode, of which its 

 basis is composed, is endowed with a very evident con- 

 tractility; the filament can contract or elongate; can ex- 

 tend itself in a straight line, or throw its length into spiral 

 curves and contorted coils; can bring its margins together, 



