mantle, 



groin of sotxl 



ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 255 



montle epithelium 

 groin oF sond 



Fig. 12-5. Artificial production of pearls. 



ward an inch or so. The blood then is 

 forced out of the foot so that it thins down 

 again and can be withdrawn from the sand. 

 The process is repeated with each step 

 (Fig. 12-4) and a wedge-shaped path is 

 left behind. 



If a clam is molested, its foot is hastily 

 withdrawn into the shell by the anterior 

 and posterior retractor muscles, and the 

 valves are slowly and tightly shut by two 

 powerful muscles, the anterior and pos- 

 terior adductors. This is the only means 

 the clam has of barring its door from in- 

 truders. To attempt to pull the valves of 

 the shell open is a nearly hopeless task, 

 unless a thin-bladed knife is first inserted 

 thrcfiigh the edge of the shell to sever the 

 large adductor muscles. The starfish, how- 

 ever, has a novel way of opening the valves. 

 It circumvents the clam, attaches its tube 

 feet to the two valves of the shell, and 

 exerts a steady pull. The pull is resisted by 

 the clam for some time, but finally the mus- 

 cles are exhausted and begin to relax ( Fig. 

 12-24). 



The two valves of the clam are hinged 

 dorsally by a ligament, which can be ob- 

 served when the adductor muscles are cut. 

 The shell itself is usually oval in shape, with 

 a blunt anterior end. Along the dorsal sur- 



face is the umbo, a bulbous structure which 

 is the oldest part of the shell. From it ap- 

 pear the concentric lines of growth, indi- 

 cating successive stages of development. 



The outer layer of the shell, the periostra- 

 cum, is produced first, then the prismatic 

 layer, and finally the innermost part, the 

 pearly layer. The periostracum is rough 

 and can resist the weak acids produced 

 by the dissolved carbon dioxide in the 

 water. The prismatic layer, which gives 

 strength to the shell, is produced from 

 crystals of calcium carbonate lying perpen- 

 dicular to the outer layer. The pearly layer, 

 the portion that interests the shell collec- 

 tor, is also composed of calcium carbonate 

 crystals that are arranged parallel with the 

 shell, resulting in an extremely smooth iri- 

 descent layer. The mantle deposits this 

 layer over any irregularities that occur, 

 either in the shell or loose particles that 

 may lodge in the mantle itself. This is the 

 origin of pearls. Foreign bodies, such as 

 grains of sand or the eggs of certain para- 

 sitic worms, sometimes become attached to 

 the mantle or lodged between the mantle 

 and the shell. In such a case, layer after 

 layer of calcium carbonate (pearl) is se- 

 creted over the particle, eventually result- 

 ing in a pearl. The Japanese produce pearls 



