BLOOD 51 



move this object without explaining to you that marvel- 

 lous play of wheels that occupies the largest part of the 

 area that you behold. As you look on the globe, you 

 observe, hanging down from the upper extremity, and 

 reaching nearly to the bottom in one direction and almost 

 from side to side in another, a transparent square veil, 

 which is indeed a flat membranous bag, having its sides 

 pretty close together, with small openings along its edges, 

 and an orifice at the bottom leading into the stomach. 



The mouth of this sac is in close connection with the 

 upper or principal orifice, and therefore receives the water, 

 which is constantly flowing in, while that aperture is ex- 

 panded. This fluid then bathes the whole interior of the 

 sac, but a portion of it escapes by the lateral openings into 

 the cavity of the body, between the sac and the mantle, 

 and is discharged through the secondary or side orifice. 



The inner surface 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, moderately rapid, ceaseless motion. 

 These black running figures, so like cogs and so well de- 

 fined 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 extremely slender 

 close-Bet hairs, as may be seen at the ends of the ovals, 



