2IO HISTOLOGY 



epithelium that also consists of two kinds of cells. The sensory cells, 

 bearing each from 60 to 100 long, stiff sensory hairs, lie apart and touch 

 each other only by the edges of their flange-like upper surfaces. Be- 

 tween them, and wedged up almost in a line with them, lie the large 

 supporting cells which seem to be, in this case, of connective-tissue origin 

 on account of their not touching the epithelial surface. An embryologi- 

 cal study of these cells would determine their origin. Lying in the 

 cavity, and just clear of the long sensory hairs, is the round statolith which 

 has an organic basis, probably of chitin, that is impregnated with car- 



FIG. 190. Statocyst of a species of Cyclas. stl., statolith; sen.c., sensory cells; sup.c., sup- 

 porting cells ; sen.r., sensory rods. 



bonate of lime. The pressure and impact of this statolith on one or the 

 other of the groups of sensory hairs must tell exactly in what position 

 the foot is lying, and, consequently, which way it is to be moved next. 

 The foot moves entirely independently of the position of the rest of the 

 body. Watch a Unio, or better, an Ensatella, that has been dug up 

 and left on the wet sand. Turn it in different positions and see how its 

 foot always attempts to go down. 



This organ should be studied in the living embryos and young of 

 Cyclas. Here it is most easily observed, and the statocyst is seen to be 

 in constant, gentle motion. We can therefore conclude that it stimulates 

 the hairs by a rhythmic impact rather than by a pressure or single impact. 



Various forms of this same organ occur through the mollusk series, 



