MOLLUSC A. 219 



earliest historic times afforded profitable fisheries. The 

 former receives its name from the absence of interlock- 

 ing teeth on the hinge of the shell, of which one or 

 more are usually present in other bivalves. 



The Scaphopoda have been regarded by some as a 

 group linking the Bivalves and the higher Mollusca.. 

 They develop from a larval stage which is in many 

 respects very similar to that of other Mollusca, but the 



Pig. 73,— The Tiisk-slien, Dentalium incrassatum. 



shell becomes tubular. The Dentalium, or Tusk-shell, 

 may be found occasionally on English shores : it is 

 exactly like an elephant's tusk in shape, and perforated 

 at the narrow end. It is more common on the American 

 coast of the Atlantic, where it is known as the tooth- 

 snail. It resembles the Gasteropoda in possessing an 

 odontophore. It has otocysts, but no eyes. Its struc- 

 ture is adapted for its habit of burrowing in sand. 



The Gasteropoda are animals all more or less resem- 

 bling the snails, and the Garden Snail {Helix aspersa, 

 sometimes called the Tortoise-shell Snail, on account 

 of the markings on the shell) is usually taken as a type 

 of the class. It belongs to the Land Snails or Pulmo- 

 natoi (snails with lungs), and is adapted for breathing 

 air; but the majority of the class live in the sea or in 

 ponds and rivers, and are adapted for breathing water, 



