158 MOLLUSCA 



References on Pearls 



Chidester, F. E. 191 5. Artificial production of pearls. Sc. Am., 



Supp. No. 2043, Feb. 1 91 5, p. 140- 

 Jordan, D. S. 1927. Mikimoto and the culture pearls. Sc. Am., Oct. 



I927> PP- 300-302. 

 Tressler, D. K. 1923. Marine Products of Commerce. N. Y. 



General Consideration of the Mollusca 



Distribution. — Mollusca are found in both fresh and salt water, 

 and on land. While they are usually free-living, they serve as 

 obligatory intermediate hosts in the development and transmission 

 of certain parasitic worms, such as the lung and blood flukes of man, 

 and are also found in association with some of the aquatic Crustacea, 



Physiology. — The soft bodies of the molluscs are protected by a 

 slimy covering frequently supported by a shell. Certain of the 

 gastropods and cephalopods lack such a shell. 



In the Lamellibranch, Pecten^ the two valves are rapidly opened 

 and closed and the animal flaps along at considerable speed. Other 

 forms like the long-necked clam {Myd) utilize their siphons in bur- 

 rowing while the majority move slowly by means of the foot. The 

 oyster is sessile in the adult condition. In certain Gastropoda there 

 is a slow wave-like muscular motion of the foot; whereas in other 

 species locomotion is accomplished by ciliary activity (Copeland ''). 

 The Cephalopoda when undisturbed move slowly by means of the 

 tail fin and sometimes by " walking " on the arms, but they are 

 capable of very rapid backward movement by jets of water ejected 

 from the " funnel " or siphon. 



The Mollusca have a well-developed body cavity usually divided 

 into two chambers, the pericardial and the visceral. Digestion is 

 facilitated by the secretions from the hepato-pancreas or " liver." 

 The Gastropoda have a highly developed lingual ribbon, the 

 " radula," which is instrumental in the penetration of hard shells. 

 Snails are able to digest cellulose without the aid of bacteria. In 

 Cephalopoda a salivary secretion is poisonous enough to paralyze 

 small crabs. The devil-fish or octopus has acid-secreting glands 

 which soften the shells of oysters at the point of drilling. Teredo 

 feeds in part on the wood from its burrows filed off by its shell. 



" Copeland, M. 1919. Locomotion in two species of the gastropod, Alectrion. 

 Biol. Bull., vol. 37, no. 2, pp. 126-138. 



