the mantle and the shell and carefully pushed 

 ahove the meat toward the adductor muscle, 

 its edge always at a sharp angle to the inner 

 surface of the valve. Actually it is preferahle 

 to move the oyster right and left, while gently 

 pressing it against the edge of the knife, rather 

 than to move the knife itself. After the attach- 

 ment of the adductor muscle to the sliell is severed, 

 the flat, right valve is lifted up until the ligament 

 breaks and the oyster is exposed in the cupped 

 valve which retains sea water. (In oyster bars 

 and restaurants raw oysters are usually served 

 on the right (flat) valve and the cupped left valve 

 is removed.) If it is necessary to open an un- 

 narcotized oyster, I prefer first to break the liga- 

 ment with a screwdriver, then to lift the valve 

 carefully and cut the adductor muscle. This 

 method reduces the chances of cutting the visceral 

 mass. 



Dissection is mucli facilitated by allowing tlie 

 tissues to harden in 3 percent formalin for at least 

 1 day. For tracing the digestive tract and the 

 blood vessels I recommend, furthermore, tlie 

 in}ection of these systems with colored moulage 

 latex. For study of the digestive tract the follow- 

 ing method gives satisfactory results: the mouth 

 of the oyster is exposed by cutting out a small 

 triangular section of both valves and pushing up 

 the underlying tissues (the mantle cap). Blue 

 or red latex diluted with aliout 20 percent water 

 is injected into the mouth through a wide glass 

 pipette slightly flattened at the tip and supplied 

 with rubber bulb. During the injection the oyster 

 is held in a vertical position. Sometimes it is 

 difficult to fill the entire digestive tract with latex 

 injected through the mouth. An additional in- 

 jection can then be made through the anus with 

 a 2-ml. capacity hypodermic syringe, preferably 

 one of metal since latex rapidly adheres to glass 

 and causes the plunger to stick. 



For injecting blood vessels through tlie ventricle 

 or auricles I prefer to use either latex diluted 

 with about .30 percent of water or vinyl resin 

 solution. Injection should be completed witliout 

 interruption in one operation, after which the 

 injected specimen is immediately placed in 5 per- 

 cent formalin in tapwater and left undistin-bed for 

 several hours or overnight to allow complete set- 

 ting of the latex or plastic. Preparations may be 

 indefinitely preserved in 3 to 5 percent formalin. 



Various cavities and chambers of the oj'ster body 

 can be advantageously studied by making plaster 



of paris casts. The valves of a live oyster are 

 forced apart by inserting an oyster knife at the 

 ventroposterior margin of the shell and gradually 

 rotating the knife until its edge is perpendicular 

 to the surface of the valves. The valves should 

 be opened very slowly to avoid tearing the ail- 

 ductor nuiscle. After a small wooden wedge is 

 inserted to prevent closing of the valves, freshly 

 made plaster of paris paste of the consistency of 

 heavy cream is injected into the cloaca and into 

 the opening of the promyal chamber. From time 

 to time the injected specimen is tapped gently 

 against the table to insure complete penetration 

 of the plaster into the smallest ramification of the 

 giU tubes. After the filling with plaster of paris is 

 completed, the wedge is removed and the valves 

 are pressed together. The preparation is left 

 undisturbed for 24 hours. After the plaster of 

 paris has hardened the shells and the soft parts 

 of the body are removed, the cast is dried for 24 

 hours at 56° C. and finally may be dipped in a hot 

 mixture of beeswax and turpentine to prevent 

 breaking of the finest ramification of the replicas. 



ORGANS UNDERLYING THE SHELL 



After the valve is removed, the body of the 

 oyster is seen to be covered with a soft membrane 

 called the mantle (figs. 71, 72). The mantle is a 

 bisymmetrical organ. Its left and right folds are 

 joined together at the dorsal edge where small 

 and slightly pigmented fold (not shown in the 

 figure) marks the position of the ligamental ridge, 

 a special organ which secretes the ligament. The 

 joint portion of the two lobes forms a cap which 

 covers the mouth and its associated structures 

 (fig. 71, m.). The remaining mantle edges are 

 free except for a point at the extreme ventral 

 margin (f.) where the two opposing lobes are 

 fused together to form a wide funnel-like channel, 

 the cloaca (fig. 72, cl.). 



The edge of the mantle consists of three pro- 

 truding fringes, two of which, the outer and the 

 middle, are beset with highly sensitive tentacles 

 (t.). The tentacles and the edge of tlie mantle are 

 commonly pigmented. 



Tlie parts of the mantle not attached to under- 

 lying organs enclose a large space filled with sea 

 water and known as the mantle cavity. Some- 

 times the open space under the mantle is referred 

 to as the shell cavity and the sea water retained in 

 it as tlie shell liquor. The space between the 

 mantle and the gills is often called the "gill cavity," 



66 



FISH AND WILDLIFE SERVICE 



