Marvels of Pond-Life. 81 



not from any want of clearness or precision in the words 

 employed, but from the complicated character of the 

 organ, and its very different appearance under different 

 aspects. To make the matter more intelligible, Mr. 

 Gosse adds, " the structure and action of an apparatus 

 of this type may be made more clear by a homely illus- 

 tration. Suppose an apple to be divided longitudinally, 

 leaving the stalk attached to one half. Let this now 

 be split again longitudinally so far as the stalk, but not 

 actually separating either portion from it. Draw the 

 two portions slightly apart, and lay them down on their 

 rounded surfaces. They now represent the quadrantic 

 masses in repose, the stalk being the fulcrum, and the 

 upper surfaces being crossed by the teeth. By the 

 contraction of the muscles, of which they are composed, 

 the two segments are made to turn upon their long axis, 

 until the points of the teeth are brought into contact, 

 and the toothed surfaces rise and approach each other. 

 The lower edges do not, however, separate as the upper 

 edges approach, but the form of the mass alters, becom- 

 ing more lenticular, so that when the toothed surfaces 

 are brought into their closest approximation, the outline 

 has a subcircular figure. It is on account of this change 

 of form that I presume the masses themselves to be 

 partially composed of muscle. - " 



These remarks, although specially made of the Rotifer 

 macrurus, are in the main applicable to all the Philo- 

 dinas, but the student must not expect to understand 

 any of the complicated gizzards of the rotifers without 

 repeated observations, and no small exercise of patience. 

 It is common to call the portions of the Philodine- 



6 



