BLOOD. 



41 



lateral opcuiiigs into the cavity of tlie body, between 

 the sac and the mantle, and is dh^charged throngh the 

 secondary, or side orifice. 



The inner surface of this transparent sac is stndded 

 with rings of a long oval figure, set side by side in four 

 rows. These rings appear to consist of a slight eleva- 

 tion 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 

 oblono; rinffs, all set round their interior with what look 

 like the cogs on a watch-wheel, dark and distinct, run 

 ning 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 op- 

 tical delusion ; they do not represent the cilia, but 

 merely the waves which the cilia make ; the cilia them- 

 selves are extremely 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 cease 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 eff'ect. A still more 

 singular circumstance is, that while in general the cili- 

 ary wave runs in the same direction in the difierent 

 ovals, there will be one here and there in which the 

 course is 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 wel] 

 as collectively. 



