390 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



A number of elementary anatomical facts are lieie Aery briefly mentioned in 

 order to make tlie explanation of sections illustratiug- the general anatomy intelligible 

 to those not already well acqnainted with it. They are also of nse in explaining a 

 number of scattered observations of my own. 



THE SHELL. 



The outer covering tn- shell of lamellibranchs is made of an organic base, im- 

 pregnated by lime, which is taken from the surrounding water. Osborn (No. 14) and 

 others have shown that the mantle lining the shell valves on their inner faces secretes 

 a soft, gummy substance, which soon becomes tough and is finally completely filled 

 with spicides of carbonate of lime. In the oyster, examined by Osborn, no prismatic 

 or mother-of-pearl layer is formed inside this, as is the case in many other forms. The 

 varying proportions of lime make a great difference in the resisting power of the 

 shell. In the shell of Mytilus eduH.s, the common mussel, there is but little lime, 

 compared with the tough, horny basis, and as a result the shell is strong and unyielding. 

 The shell of Verms mercewaWa, the "little- neck clam "or "quahog,"as it is often called 

 on the Atlantic coast, is, on the other hand, made almost entirely of lime. Though 

 the shell is thick and heavy it breaks into many fragments, like a piece of porcelain, 

 when struck a severe bh)w. The thickness of the shell of many lamellibranchs 

 depends greatly upon the amount of lime held in solution by the water in which it 

 exists. 



It is characteristic of this class of the Mollusca that the shell is made of two 

 independent halves, called valves, which are joined to each other by a ligament. This 

 is tough and rubber-like, and between it and the concavity of the shell is the hinge, 

 where the valves, in touching each other, form a fulcrum. In many cases one valve 

 is hollowed out at this point and the other has a corresponding projection, or per- 

 ha])s many of these, each fitting into a hollow. These relations may be seen in Fig. 

 96, PI. XCIV, where l(j is the ligament ami h<i the hinge. The adductor muscles {aa and 

 pa), on contracting and closing the shell, cause, through the action of the fulcrum at 

 the hinge, a stretching of the ligament. When these muscles are again relaxed the 

 ligament contracts automatically, as would a piece of rubber, and opens the valves 

 at the opposite extiemity. 



In the oyster there is no distinct hinge, but the ligament is made of two parts, a 

 central, thick, and elastic portion (Fig. 97, PI. xciv, Ig), and above and below this a 

 slight ridge. The shell projects slightly at these points and may help in functioning 

 as a hinge. The valves of the shell are generally equal to one another in size and 

 shape. In the oyster the left valve is the larger one; it is nuich heavier and is suffi- 

 ciently hollowed out to contain the whole of the soft parts (Fig. 97, PI. xciv), while the 

 right valve is smaller and almost flat. The animal is attached l)y the former. 



The outer surfaces of the shell are generally marked by concentric lines of growth, 

 and along its edges may be found the horn -like cuticle, secreted by the mantle edge. 

 This cuticular covering of the <mter surface of the shell is often thin and may be lost, 

 except at the edges. In the case of Solenomi/a it is very thick, covering tlie whole 

 shell from sight and extending some distance beh)w its ventral edge. The umbo, a 

 rounded prominence on the dorsal side of many shells, is well marked in Venus (Fig. 

 90, PI. xciv). 



