OF PROTOZOA TO BILATERALITY. 



175 



of this division to Hlaterality. I will endeavor to show how the 

 obliquity of the spiral is resolvable into a bilateral form. A 

 strip of paper shaped into a long parallelogram and folded 



Fig. 101. 



Fig. 102. 



,/l 



lengthwise along the middle will represent the idea of bilateral- 

 ity, one half forming the right and the other the left side. Now, 



for the sake of convenience in winding 

 it up, cut it lengthwise along the mid- 

 die, (fig. 101,) except for a short dis- 

 A tance at one end, which may represent 

 the head. Then, the two halves being 

 folded together as before, it may be 

 wound up in a spiral with the uncut 

 end, i. e. head, on the outside. This 

 may be wound either to the right or 

 to the left, according to circumstances. 

 When coiled in a long-drawn-out 

 spiral, (fig. 102,) we have the form of 

 Dysteria typified, or, twisted a little 

 closer, a Spirostomum, then still more, 

 a Paramecium and Pleuronema, when 

 made into a nearly flat spiral, (fig. 103,) 

 a Stentor, and finally, when wound up 



Fig. lOi. 



Fig. 104. Z ' oolTiamnium arbuscula. Ehr. 200 diam. A stem with two 

 individuals. A is fully expanded ; B is partially shut up, by the infolding of 



