194 AQUATIC MICROSCOPY FOR BEGINNERS. 



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

 produce currents in the water that sweep away small 

 objects with some rapidity. 



Two or more small black or reddish eye-spots are 

 often present near the front border, and, in some of 

 these worms, may be rather 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 location of the mouth varies widely in-the differ- 

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

 some point nearer the center of the ventral surface of 

 the body, or even close to the posterior margin. It is 

 usually large and expansile, 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, usually extending from the pharynx to the pos- 

 terior border -of the animal. In sonie it is simply a 

 great sack, receiving all that the mouth and pharynx 

 turn into it; in others it divides into many branches 

 whose terminations may be seen near both sides of the 

 body. The stomach seldom has a posterior opening, 

 for as a rule, there is no intestine. 



After the nutriment has been digested and absorbed 

 from the food, the insoluble remains must be ejected 

 through the mouth. It is no unusual sight, therefore, 

 to see one of these ciliated worms vomit up a mass of 

 indigestible and empty Rhizopod shells, Rotifer cara- 

 paces, together with many unrecognizable particles 



