180 MICROSCOPY FOR BEGINNERS. 



are soft and flexible, and some are quite changeable in 

 shape, having the power to lengthen themselves, to ex- 

 tend the posterior border into a short projection, or to 

 narrow the front into an apology for a head. Some, 

 however, have the front part naturally prolonged into a 

 short snout. They are usually brownish and almost 

 opaque, the opacity being increased by the large amount 

 of food commonly present in the stomach. 



The cilia clothing the entire surface are visible only 

 under a high power. The result of their motion, how- 

 ever, can be seen with the one -inch objective, as they 

 produce currents in the water that sweep small objects 

 quite rapidly away. 



Two or more small black or reddish eye -spots are 

 often present near the front border, and in some of 

 these worms may be quite complicated in structure, 

 having a covering that may not inappropriately be 

 called a cornea, a refracting body corresponding to a 

 crystalline lens, pigmentary or coloring matter, and a 

 nerve. 



The position of the mouth varies widely in the differ- 

 ent families. It may be at or near the front border, at 

 some point nearer the centre of the body, or even close 

 to the posterior margin. It is usually large and expan- 

 sile, and is often followed by a large and very muscular 

 organ called the pharynx, which some of the worms can 

 protrude, and with it snap up their living prey. The 

 lining of the pharynx may be finely ciliated. 



The stomach occupies the largest portion of the body, 



