GASTEROPODA 81 



its shell partly exposed. Along the Atlantic coast people use the Mya 

 or Ve7ius for their " clam-bakes." Many hundred bushels are used every 

 year for this purpose. 



The razor-shell clams have similar habits. They are concealed in 

 vertical holes in the sand with the posterior end of the shell uppermost. 

 They have a powerful club-shaped foot, and can dig so rapidly that unless 

 one approaches very cautiously they escape from view. They seem to be 

 sensitive to light and to the " jar " made by approaching footsteps. 



The borer (Pho'las) has its brittle but very hard shell marked like a file, 

 with which it bores into the hardest rocks. The united siphons are longer 

 than the rest of the body. Some forms are phosphorescent, emitting 

 bluish-white light. 



The ship-worm (Tere'do) (Fig. 60) , another borer, works into wood, doing 

 much damage to ships in the tropics. The larva enters the wood when it is 

 extremely small and enlarges the tunnel as it grows. The wood which 

 it excavates is not used for food, but is carried off by the excretory siphon. 

 Its food, which consists of microscopic organisms, is brought in by the 

 currents. The amount of damage these borers do seems incredible. They 

 completely honeycomb the hull of a wooden vessel. The best protection 

 against them is the sheathing of the hull with copper. Palmetto is the 

 best resistant among woods. The ship-worms caused the destruction of 

 a dam in Holland, threatening destruction to the country. Their dis- 

 persal is wide, since they are carried all over the world in the floating wood 

 which they attack. 



CLASS II. GASTEROP'ODA 



These are asymmetric, usually univalve moUusks, and the 

 head region bears either one or two pairs of tentacles. As in 

 the snail (Fig. 61), the eyes are borne either at the bases or at the 



Fig. 61. — A snail. (After Tenney.) 



tips of the tentacles. The shorter tentacles are probably organs 

 of smell. The head contains the mouth, in which is the tongue, 

 covered by the radula, a ribbon-like organ supplied with 

 chitinous teeth and used for rasping the food. 



The mantle is not divided into two parts as in the mussel, but 

 unites around the neck, leaving but a small respiratory aperture 

 6 



