406 APPLIED BIOLOGY 



Shell. (L) Examine some bivalved shells ; some empty, and one 

 with the clam inside. Note the following : 



(1) Hinge, which unites the valves. It has an elastic band that 

 pulls the valves apart when the muscles relax, as when the animal 

 dies. Interlocking ridges and tooth-like structures form the hinge- 

 joint in some species. 



(2) The hump (umbo) is an elevation on the hinge line of the 

 shell. It is the oldest part of the shell. 



(3) Concentric lines extend outward from the umbo as a center. 

 These are lines of growth, and represent successive additions to the 

 shell. 



(4) The fleshy tissue extending along the free edge of the shell 

 between the two valves is the thickened edges of the two folds of 

 the mantle, each of which lines a valve and secretes new shell mate- 

 rial at the edge as indicated by the lines of growth. 



(5) On an empty pair of valves, note the two pairs of scars where 

 were attached the two muscles (named adductors) which hold the 

 valves together during life. 



(6) The inner pearly lining of the shell is secreted by the mantle. 

 A small particle of sand introduced between the shell and mantle 

 will become coated with pearly substance. Small particles of pearly 

 substance may become the center of solid pearls. The middle calca- 

 reous and outer horny layers of the shell are secreted at the edge of the 

 mantle. 



(7) (Z>) Drop a piece of shell into strong acid to demonstrate 

 its calcareous nature. The gas which causes the effervescence is 

 carbon dioxide. (How could you prove it ?) Heat a shell in a hot 

 flame or stove for some time and it becomes a mass of lime. 



(8) The hinge is on the dorsal side of the animal, the opposite 

 or free edge of the shell is ventral. The siphon or "neck" is poste- 

 rior, and the umbo and foot are near the anterior end. Holding a 

 shell with hinge upward and the posterior end towards you, the right 

 side of the clam is at your right hand. 



(9) Currents of Water. (D) If a living clam is available in an 

 aquarium, drop (with a pipette) some very muddy water near the 

 siphon (popularly called "neck"), and observe directions of currents 

 (see Fig. 141). These currents supply both oxygen and minute 

 organisms as food in the ingoing (inhalent) current, and remove 

 excretions by the outgoing (exhalent) stream. Bivalved mollusks 

 have no other method of getting food and oxygen. Some of them 

 burrow in mud, wood, and even soft rock, and spend their lives 

 connected with the open sea only by means of the siphon, which 



