INFUSORIA. 147 



back along its body ; so that it formed a kind of movable keel, 

 when the little creature glided through its watery element, or 

 was used to sway it from side to side, or oftentimes to raise it 

 up on its tail by forming a prop, as we see it in this other figure 

 (fig. 88, B). The motory or propelling power, on the other hand, 

 is restricted, at least in the greatest measure, to another kind 

 of vibratile cilia. These are very short, and are crowded to- 

 gether in great numbers in a broad furrow or depression, (c/, cl\) 

 which extends over half the length of the body, along its infe- 

 rior, middle line. When the body is turned over, and the ante- 

 rior end retracted and swelled out sideways, the furrow becomes 

 quite conspicuous, (fig. 88, B,) and the extent of the group of 

 minor cilia is easily ascertained. They are very minute and in 

 constant motion, propelling the body backwards and forwards, 

 up and down, to the right or left, according as it is steered by 

 the trailing lash which extends along its length. Thus it is, that, 

 although similar in form, a diversity of functions is laid upon 

 these three kinds of cilia that amounts to the most marked spec- 

 ialization, through the simplest means ; in fact, so simple that the 

 eye cannot detect them in any form beside that of proportion 

 and position, and certainly not in the intimate structure of these 

 bodies. The whole body, too, possesses a flexibility and exten- 

 sibility scarcely inferior to its cilia ; at one moment it is darting 

 through the water, sharp as a lance at both ends, and at the next 

 it is as round as a ball, or worming its way through tortuous 

 passages with every possible degree of flexure short of actually 

 tying itself into a knot. 



When, now, I turn your attention to one of the next succeed- 

 ing higher forms, this one here, (fig. 89,) for instance, Ceratium, 

 some of you may perhaps recognize in the one which we have 

 just studied a transition from those, like Chlamidomonas (fig. 

 87) and Euglena (fig. 86), which have but one kind of vibratory 

 lash to those which possess two forms of such organs special- 

 ized to the highest degree. In this new subject the smaller cilia 

 (fig. 89, w, w l ) are methodically disposed in a linear series 

 along the edge of the circumambient annular furrow which 



