BLOOD. 37 



serve, hanging down from the upper extremity, and reach- 

 ing 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 expanded. 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 



