PROCURING OBJECTS. 53 



This quantity suffices to fill a half-pint mug, and to keep it at the same 

 level during the season. Then tie up a small portion of hay, about the 

 size of the smallest joint of the little finger, trimming it so that it may 

 not occupy too much room in the mug, and place it in the water ; or the 

 same quantity of green sage leaves will do. Every ten days the decayed 

 portion should be gently removed with a piece of wire, and a fresh 

 supply substituted. By either of these methods a good supply of wheel 

 animalcules can always be kept up. 



53. POLYPI. These animals, which are exceedingly interesting 

 microscopic objects, are to be found upon all sorts of aquatic plants 

 upon branches of trees, pieces of board, rotten leaves, stones, and 

 other substances that lie in the water. They should be sought for in 

 the corners of ditches, puddles, and ponds, being generally driven into 

 these with the pieces of wood or leaves to which they have attached 

 themselves. They are seldom to be met with in winter ; about the 

 month of May they begin to appear and increase. They are generally 

 found in waters which move gently, for neither rapid streams nor stag- 

 nant waters ever abound with them. I have drawn attention to these 

 animals, because they are, probably, the most convenient for viewing 

 the internal organization of animalcules. Their usual food assimilates 

 so closely in colour to themselves, that it is impossible, under ordinary 

 circumstances, to perceive the form of their digestive functions. Mr. 

 Trembely, and, subsequently, Dr. Ehrenberg, have pursued the plan 

 of feeding them with minutely divided solutions of coloured substances, 

 such as indigo, carmine, and sapgreen, and by this means have ascer- 

 tained the form of the digestive cavities in animalcules. It is essential 

 that whatever colouring matter we employ should be pure and free 

 from impurities, and that it be only mechanically not chemically 

 soluble in water. 



54. MODE OF SELECTING AQUATIC LARV.* AND OTHER SMALL 

 ANIMALS. It is usual, for the purpose of thoroughly examining them, to 

 obtain a quantity of the animalcules, found in stagnant pools or gentle 

 streams, and to preserve them in convenient vessels. Although they are 

 usually visible to the naked eye, yet it is difficult to select a specimen 

 for examination, their organization being so delicate as not to allow of 



