74 



of these the relatively large, greenish, slipper-shaped animals are Paramecia. 

 Note carefully the general shape of the body. Are the two ends different in 

 form? Is one end always directed forward in swimming? If so, which end? 

 Does the animal have a definite permanent shape? 



Watch and describe in your notes the swimming movements. Observe that 

 the animal when swimming in a free field revolves upon its long axis; also it 

 seems to swerve from side to side. What is the cause of the rotation and the 

 swerving and what is the real path of the animal (see Hegner, p. 65)? 



By this time the student will probably have observed that the animal is 

 not symmetrical and cylindrical ' but that the anterior half is deeply concave 

 as if a large slice had been cut out of it. This concave depression is called the 

 oral groove. It is best seen by watching the animals as they revolve. The side 

 of the Paramedum on which the oral groove is located is the ventral or oral 

 side; the opposite side, the dorsal or aboral surface; and right and left are then 

 easily determined. (In Hegner, Fig. 23, p. 60, the labels R and L have been 

 interchanged.) Observe the freely moving animals carefully until you have 

 obtained a correct idea of the width, length, and direction of slant of the oral 

 groove. How wide is it compared with the anterior end? Does it slant from 

 right to left or left to right? (Remember that as the animal is transparent you 

 cannot determine whether you are looking at the upper or under side of it, except 

 by the use of the focusing screw.) How far posteriorly does it extend? 



Make an outline drawing of the animal from the ventral side, about four 

 inches long, showing its correct shape and proportions and correct appearance 

 of the oral groove. Have the drawing approved by the assistant before proceed- 

 ing. The details of structure are later to be entered upon this outline. 



2. Detailed structure. This study must be made with the high power, a 

 proceeding usually fraught with difficulty for the student because the animal 

 will not stand still. One or all of the following methods of quieting the animals 

 must therefore be employed and will ordinarily prove successful: (i) Always 

 mount the animals with a piece of scum or other solid material from the culture. 

 They will generally remain quiet around this, or around air bubbles, or often 

 near the edge of the cover glass. This is the most satisfactory method of study- 

 ing Paramedum, as the animals remain in a normal condition throughout. 

 Every student should try this first. (2) Withdraw the water from the slide 

 by applying a piece of filter paper to the edge of the cover glass, so that not 

 enough water remains for the animals to swim in. Under these conditions, 

 however, the normal shape and appearance of the animals is lost, and they soon 

 burst and die so that observations must be completed quickly. (3) Mount a 

 drop or two of Paramedum on a slide and place in the center a small drop of a 

 very dilute solution of formaldehyde. Cover. The animals near the formalde- 

 hyde will soon slow down and finally become entirely motionless; those farther 

 away will eventually also succumb. For a time, the motionless animals retain 



