THE PLAY OF THE GILLS. 243 



you look on the accompanying figure ; the digestive 

 organs lying beyond and beneath it. The inner sur- 

 face of this transparent sac is studded with rings of a 

 long-oval figure, set side by side in four rows. These 

 rings appear to consist of a slight elevation of the 

 general membranous surface, so as to make little 

 shallow cells, the whole edges of which are fringed 

 with cilia, whose movements make waves that follow 

 each other round the course in regular succession. 

 In truth it is a beautiful sight to see forty or more of 

 these oblong rings, all set round their interior with 

 what look like the cogs on a watch-wheel, dark and 

 distinct, running round and round with an even, mo- 

 derately rapid, ceaseless motion. (See fig. 2). These 

 black running figures, so like cogs and so well defined 

 as they are, are merely an optical delusion ; they do 

 not represent the cilia, but merely the waves which 

 the cilia make ; the cilia themselves are exceedingly 

 slender, and close-set hairs, as may be seen at the ends 

 of the ovals, where a slight alteration of position pre- 

 vents the waves from taking the tooth-like appearance. 

 Sometimes one here and there of the ovals ceases to 

 play, while the rest continue ; and now and then, the 

 whole are suddenly arrested simultaneously as if by 

 magic, and presently all start together again, which 

 has a most charming efi'ect. But what struck me as 

 singular was that while in general the ciliary wave 

 ran in the same direction in the diff'erent ovals, there 

 would be one here and there, in which the course was 

 reversed ; and I think that the animal has the power 

 of choosing the direction of the waves, of setting them 

 going and of stopping them, individually as well as 

 collectively. 



