THE ANATOMY OF THE OYSTER. 713^ 



" In front of the gills, that is, between them and the hinge, there are four fleshy flaps — the 

 lips — two on each side of the body. They are much like the gills in appearance, and they are 

 connected with each other by two ridges which run across the middle of the body close to the 

 anterior end, and between these folds is the large oval mouth, which is thus seen to be situated, 

 not at the open end of the shell, but as far away from it as ijossible. As the Oyster is immovably 

 fixed upon the bottom, and has no arms or other structures for seizing food and carrying it to the 

 mouth, the question how it obtains its food at once suggests itself. If a fragment of one of the gills 

 is examined with a microscope it will be foimd to be covered with very small hairs, or cilia,, 

 arranged in rows. Each of these cilia is constantly swinging back and forth, with a motion 

 something like that of an oar in rowing. The motion is quick and strong in one direction and 

 slower in the other. As all the cilia of a row swing together, they act like a line of oars, only they 

 are fastened to the gill, and as this is immovable, they do not move forward through the water, but 

 produce a current of water in the opposite direction. This action is not directed by the animal, 

 for it can be observed for hours in a fragment cut out of the gill, and if such a fragment be supplied 

 with fresh sea-water, the motion will continue until it begins to decay. While the Oyster lies 

 undisturbed on the bottom, with its muscle relaxed and its shell open, the sea-water is drawn on 

 to the gills by the action of the cilia, for although each cilium is too small to be seen without a 

 microscope, they cover the gills in such great numbers that their united action produces quite a 

 vigorous stream of water, which is drawn through the shell and is then forced through very small 

 openings on the surfaces of the gills into the water-tubes, inside the gills, a nd through these tubes 

 into the cavity above them, and so out of the shell again. As the stream of water passes through 

 the gills the blood is aerated by contact with it. The food of the Oyster consists entirely of minute 

 animal and vegetable organisms and small particles of organized matter. Ordinary sea-water 

 contains an abundance of this sort of food, which is drawn into the gills with the water, but as the 

 water strains through the pores into the water-tubes, the food particles are caught on the surface 

 of the gills by a layer of adhesive slime which covers all the soft parts of the body. As soon as 

 they are entangled the cilia strike against them in such a way as to roll or slide them along the 

 gills toward the mouth. When they reach the anterior ends of the gills they are pushed off and 

 fall between the liiis, and these again are covered with cilia, which carry the particles forward 

 until they slide into the mouth, which is always wide open and ciliated, so as to 'draw the food 

 through the oesophagus into the stomach. Whenever the shell is open these cilia are in action,, 

 and as long as the Oyster is breathing a current of food is sliding into its mouth. 



"The cilia; and particles of food are too small to be seen without a microscope, but if finely 

 powdered carmine be sprinkled over the gills of a fresh Oyster, which has been carefully opened 

 and placed in a shallow dish of sea- water, careful observation will show that as soon as the colored 

 particles touch the gills they begin to slide along with a motion which is quite uniform, but not 

 much faster than that of the minute-hand of a watch. This slow, steady, gliding motion, without 

 any visible cause, is a very striking sight, and with a little care the particles may be followed up 

 to and into the mouth. 



"In order to trace the course of the digestive organs, the visceral mass may be split with a 

 sharp knife or razor. If the split is pretty near the middle of the body, each half will show 

 sections of the short, folded oesophagus, running upward from the mouth, and the irregular 

 stomach, with thick, semi-transparent walls, surrounded by the compact, dark-greenish liver. 

 Back of the liver and stomach the convoluted intestine will be seen, cut irregularly at several 

 points by the section. 



"There are no accessory organs of reproduction, and the position, form, and general appear- 



