7 8 THE MICROSCOPE. 



the dorsal side, by innumerable and invisible lashes. I said invisible, 

 for it requires a high power of the microscope to detect this mystic 

 mechanism. Analogy compels us to believe that it is a similar cili- 

 ated movement which carries the food down the alimentary canal. 

 A little way from these lips, or mouth, is the oesophagus, and a little 

 lower, right in the middle of this dark mass, which is the liver, is the 

 stomach. Notice its irregular shape. Now follow the course of the 

 intestine. It extends fully half way to the posterior end of the ani- 

 mal, and then in a surprising manner doubles itself, and extends 

 back again until it almost reaches the oesophagus, where it again re- 

 curves, returning nearly to the posterior of the animal, or opening 

 of the shell, where its vent stops at this depression, or region, which, 

 if not organically, is functionally the cloaca. How beautiful this 

 arrangement, for the spot at which the vent stops is directly in the 

 egress current of the clean, but effete water, which is leaving the 

 shell and returning to the sea. Of course you must notice the sig- 

 nificance , of this irregular stomach, and this long intestinal tract, 

 viz.: for the more complete absorption of the digestible parts of the 

 food. But let me direct your attention to another feature intended 

 to increase the surface of the alimentary canal, and so to increase 

 the area of osmotic action. If I cut across this intestinal duct, get- 

 ting a transverse section, I find that the form inside differs from that 

 outside, which is cylindrical. Now the inner wall of this tube is 

 convolute. It is much as if a smaller tube was soldered to the inner 

 wall along'its whole length, thus greatly enlarging the absorbing 

 surface. Hence a faecal cast would not be that of a solid cylinder, 

 but in cross section would present a form like that of a dime with a 

 three-quarters of a circular hole punched at its edge. Now, when 

 you consider that in this instance the osmotic action on the food is 

 equal to the surface over which it passes, and that this is estimated 

 by multiplying the length of the canal by the perimeter of the inner 

 wall, plus the perimeter of the convoluted wall, you see how great 

 the surface is, hence how complete is the ingestive system, even in 

 an oyster. 



3. Respiration. — Perhaps even more beautiful is the respiratory 

 system. Here, in the posterior part of the animal, so as to be near 

 the water, are the branchial plates, or gills. Like the palpi, they are 

 in two pairs, one pair for the left side and one for the right. They 

 comprise an infinity of little tubes, so small that the unaided eye 



