When it contracts the shell closes, but upon 

 its relaxation the elastic properties of the 

 ligament located at the hinge force the valves 

 apart. Even superficial examination of this 

 muscle shows that it consists of two different 

 parts (fig. 2): a sennitranslucent part near 

 the hinge, and an opaque, white part surround- 

 ing the first part. Microscopic examination of 

 the tissues composing these two parts of the 

 muscle also shows that their fibrils differ in 

 size and structure. 



Physiological studies of the muscle have 

 shown clearly that the two parts perform 

 distinct functions. The translucent area can 

 contract very quickly but, on the other hand, 

 it soon tires and is compelled to relax. There- 

 fore, it could be called appropriately the quick 

 muscle. The opaque portion, however, con- 

 tracts relatively slowly but can remain con- 

 tracted for long periods while expending little 

 energy. Furthermore --and this is important-- 

 when the opaque portion of the adductor muscle 

 contracts, it becomes locked as if by a ratchet: 

 hence, it is known as the catch muscle. This 

 long- sustained contraction may be caused by a 

 continuous discharge of nerve impulses. The 

 ability of the oysters to stay closed for several 

 days or even weeks is extremely useful be- 

 cause it helps them survive unfavorable condi- 

 tions, such as freshets or temporary pollution 

 of the water. 



Shell movements of an oyster caused by 

 contraction or relaxation of the adductor 

 nnuscle can be recorded easily by several 

 means. One method consists of recording the 

 shell motions on a kymograph (an apparatus 

 that has a recording p>en and paper mounted 

 on a drum which rotates at a constant speed). 

 Studies of shell movements are now commonly 

 used to observe behavior of oysters under 

 normal and abnormal conditions. For example, 

 records obtained on shell movements of oysters 

 exposed to different concentrations of pol- 

 lutants in sea water showed the response of 

 the oysters to their presence and the level of 

 tolerance of these substances. Usually, in the 

 presence of pollutants or during unfavorable 

 physical changes in the environnnent, the 

 rh^-thm of opening and closing of the shells 

 is changed, and movements of the valves 

 become irregular. If the concentrations of 

 harnnful substances become too strong, the 

 shells of the oysters remain closed. 



The main body of the oyster lies between 

 two folds of the mantle which is attached to 

 it. Under the mantle at the anterior end of the 

 body, nearest to the hinge, are four thin lips, 

 or palps (fig. 2), After the lips are four rows 

 of sickle-shaped organs known as the gills, 

 arranged one below the other like pages in a 

 book. The gills extend almost from the mouth, 

 which IS hidden under the palps, to about two- 

 thirds of the distance around the body. At 

 their end the mantle lobes of the two sides 

 are united, and this union divides the nnantle 



cavity into a large, inhalant chamber containing 

 the gills, and a much smaller, or exhalant, 

 chamber. The water enters the inhalant cham- 

 ber and leaves the oyster body by the exhalant 

 chamber. Therefore, the gills form a com- 

 plete partition dividing the mantle cavity. The 

 only passage from one chamber to another is 

 through the fine interstices or openings between 

 the cross-connected filannents of the gills. 



As Yonge indicates in his book, the precise 

 region where the water enters the body of the 

 oyster is controlled by the margin of the 

 mantle. Usually this margin separates only 

 in the central region of the inhalant chamber 

 so as to concentrate the inflowing water and 

 increase its speed. 



The gills of an oyster, and of many other 

 bivalves, are complex organs principally con- 

 cerned with respiration and feeding. In gen- 

 eral, these gills may be compared to a fine 

 sieve. The openings of the sieve, called ostia, 

 are surrounded by structures resembling mi- 

 croscopic whips, or cilia, which beat inward 

 in an orderly manner, producing a current of 

 water that passes through the pipelike struc- 

 tures inside the gills and is finally discharged 

 through the exhalant or cloacal chamber. 



Surfaces of the gills also are covered with 

 cilia arranged in definite rows. They lash 

 continuously, creating a current of water, 

 and push the microscopic algae and other 

 small particles toward the edges of the gills. 

 The material that settles on the gill surfaces 

 is entangled in mucus secreted by special 

 cells of the gills and is carried gradually 

 toward the mouth. Before the material is 

 swallowed it is sorted, first on the gills 

 themselves but especially in the region of 

 the palps, A portion of the material may be 

 rejected in the form of pseudofeces, which 

 consist of mucus secretions producedby oyster 

 gills in which small marine algae, detritus, 

 and particles of turbidity-causing materials 

 are embedded. When food organisms or tur- 

 bidity-creating particles, such as silt, are too 

 numerous, the oyster may reject as pseudo- 

 feces the largest portion of material collected 

 on its gills. In general, oysters, like many 

 other bivalves, feed most effectively in rela- 

 tively clear water. 



Scientists have found that the rate of water 

 pumping is affected by several factors, in- 

 cluding temperature, salinity, turbidity, and 

 the presence in the water of various chemical 

 substances. Most Eastern oysters stop feeding 

 and hibernate when the water temperature 

 decreases to about 41° F. Some adult Long 

 Island Sound oysters, however, pump water 

 at temperatures as low as 34° F, but, as a 

 rule, the rate of pumping remains low as long 

 as the temperature of the water is below 46° 

 or 47° F, Within the range of about 47° to 

 61° F. the rate rapidly increases, but between 

 61° and about 82° F, the increase is compara- 

 tively slow. Between 83° and 90° F, a further 



