542 Animal Biology 



such foreign substances as grains of sand, fragments of tissues, bits of 

 shell, eggs, worms, small Crustacea, and similar objects {Figs. 118, 120, 

 and 121). In some instances foreign bodies are artificially introduced 

 into the bodies of the mollusks and the layers of mother-of-pearl are 

 added in concentric lavers bv the mollusk. Should we consider this an 

 artificial or natural method of pearl formation? 



The internal shell of the cuttlefish (class Cephalopoda) is sold as cuttle 

 bone. This is used for food for birds and is porous and made largely 

 of lime. Cuttlefishes or Sepias furnish the ingredients for sepia ink, 

 which is used in art. India ink is made from the ink bags of fossil cut- 

 tlefishes. Ground cuttle bone is called "pounce" and is used by drafts- 

 men to prevent blotting and used in medicine as an antacid. 



The devilfish or Octopus (Fig. 124) sometimes attacks man, although 

 not as frequently as once supposed. This type of mollusk is used for hu- 

 man food. 



Snails are of medical and sanitary importance because they act as hosts 

 to larval flatworms which may eventually become parasitic for higher 

 vertebrate animals, including man. The European land snail (Helix 

 pomatia) (Figs. 116, 117, and 119) is imported in large numbers for 

 food and laboratory purposes. The snail is a great source of food in 

 certain European countries, taking much the same status as the oyster in 

 this country. In spite of many attempts at introduction, it has never 

 pleased the palates of the American populace. 



The giant land slug (class Gastropoda) is used by Indians of South 

 America to manufacture the so-called "bird lime" to capture humming- 

 birds. 



The boring snail (Natica) destroys other mollusks by boring into their 

 shells and eating them. The borer (Pholas) (class Pelecypoda), because 

 of its filelike shell, is able to bore through concrete and rocks, thus being 

 of importance to shipping industries. The wood-boring shipworm 

 (Teredo navalis) (class Pelecypoda) is able to destroy the wood of ships, 

 wharves, and piles unless protected by concrete or creosote (Fig. 122). 

 They have been known to bore for a distance of more than two feet. 

 Chitons (class Amphineura) are used for bait and human food (Fig. 115). 



The squid (class Cephalopoda) is used for food. Squid oil is used by 

 the Chinese as medicine and is used elsewhere for lubrication purposes 

 (Fig. 123). The so-called "pen" is a thin, internal, chitinous shell em- 

 bedded along the dorsal side. The ink sac discharges an inky secretion 

 into the water to confuse enemies. 



