130 BULLETIN OF THE BUREAU OF FISHERIES. 



ray of light are characterized by thin, colorless shells, a highly developed tactile sense, 

 and the absence of visual organs. 



The length of life and age of attaining sexual maturity vary considerably for different 

 mollusks. Mytilus reaches the adult stage in one year. The fresh-water mussels, Ano- 

 dontidse, do not reach sexual maturity until they are 5 years old. Some mollusks, nudi- 

 branchs, and the cephalopod, Rossia, appear to live for one year only, while others, as 

 Mytilus and the oyster, may live 10 or more years; the periwinkle, Littorina, has been 

 known to attain an age of 20 years in captivity, and the Anodontidae, which are remark- 

 able for their long life, may reach an age of 25 or 30 years. 



The Mollusca is an old group whose fossil representatives are found in all Paleozoic 

 deposits upward. As a group it has met the changing conditions of the world most 

 successfully, as is clearly demonstrated by its present abundance and wide distribution. 

 More than 28,000 living Mollusca have been described up to the present time, more than 

 half of which are Gastropoda. 



The economic importance of the Mollusca is very great indeed. The group includes 

 species of negative as well as of positive value. In the former class may be mentioned 

 the so-called shipworm, Teredo navalis, a boring lamellibranch whose habits are ex- 

 tremely destructive to the bottoms of wooden ships, to wharf piles, and to other sub- 

 merged wooden objects, which are riddled by its borings. To prevent the destructive 

 inroads of the shipworm it is necessary to incase the bottoms of wooden ships with a 

 metal sheath and to coat such wooden objects as spars, buoys, etc., with verdigris paint 

 periodically every six months. 



Among the gastropods are found many voracious species armed with rasping 

 organs against which few shellfish are safe. They prey upon many species valuable 

 to man, such as oysters, clams, scallops, mussels, etc., by boring holes through their 

 shells and literally eating them alive. The destructive ravages of these snails on the 

 commercial species of mollusks amount to many thousands of dollars yearly. The 

 Cephalopoda are also carnivorous animals of very active and voracious habits. They 

 dart into the schools of young fishes and feed upon them in great numbers. Young 

 lobsters and other small crustaceans often fall prey to them. 



Molluscan species of positive value to man are numerous and represent every 

 class except the Amphineura. Most of the cephalopods are good to eat and are uti- 

 lized extensively as food in some countries. Although not used as such in the United 

 States, there is no reason why they should remain a neglected food product. Squid 

 is the most valuable bait known in the cod fisheries and for this reason often brings 

 fancy prices. When abundant it is used for fertilizer. The cuttlefish furnishes the 

 cuttle bone which is used as a food for canary birds, and formerly its inky secretion 

 was sold as India ink or sepia, which was used for drawing purposes. 



The gastropods include species of food value, as, for example, the large, edible 

 snail of Europe, the periwinkle, Littorina littorea, which is eaten by the ton in London, 

 but, as yet, remains unknown as a food in this country. The abalone of our western 

 coast is beginning to be appreciated as a food through the influence of the Chinese, 

 who have developed the fishery into a business worth many thousands of dollars annu- 

 ally. Its shell is remarkable for its great beauty and was formerly used by the Indians 

 for making their money. In Europe it is used for making buttons, studs, and buckles 

 and for decorating purposes. 



