OYSTERS AND METHODS OF OYSTER-CULTURE. 277 



by the bodj', or A'isceral mass, wliicli is made up mainly of the light-colored rei^ro- 

 duetive organs and the dark-colored digestive organs, packed together in one 

 continuous mass. 



If the oyster has been opened very carefully, a transparent, crescent-shaped space 

 vrill be seen between the muscle and the visceral mass. This space is the pericar- 

 dium, and if the delicate membrane which forms its sides be carefully tut away, the 

 heart, re and an, may be found without any difficulty lying in this cavity and pulsat- 

 ing slowly. If the oyster has been opened roughly, or if it has been out of water for 

 some time, the rate of beating may be as low as one a minute, or even less, so the heart 

 must be watched attentively for some time in order to see one of the contractions. 



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

 flaps — the lips, p, 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, m, which is thus seen to be situated, not at the open end of the shell, 

 but as far away from it as possible. 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 found to be covered with very 

 small hairs, or cilia, arranged in rows, plate viii, fig. 3, c. 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 pro- 

 duces 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, and 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 coutact 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 aii 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 iiarticles 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 i)ushed off" and fall between the lips, 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 

 tJirough the n-sophagus 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 jdaced 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 

 miuute-hand of a watch. This slow, steady, gliding motion, without any visible 



