BLOOD. 37 



serve, 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 connexion 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 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 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 close- 

 set hairs, as may be seen at the ends of the ovals, where a 

 slight alteration of position prevents 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 simultane- 

 ously, as if by magic, and presently all start together again, 

 which has a most charming effect. A still more singular 



