100 THE STORY OF ANIMAL LIFE 



there are even distinctions between the tongues 

 of different species in the same genus. Conse- 

 quently some authorities on shell-fish prefer to 

 classify them by their tongues, a classification 

 which for the most part holds good. So char- 

 acteristic is the tongue of the Gasteropod, that 

 when new animals have turned up which were 

 difficult to classify by means of the structure of 

 the body, they have been finally recognised as 

 Molluscs, somewhat related to the snails, by the 

 tongue. This file-like tongue-ribbon of the snails 

 is often called the Odontophore or Tooth-Carrier; 

 sometimes the part which actually bears the teeth 

 receives the name of the radula. 



The snail and its relative, the slugs, belong to 

 the Pulmonate (i.e. air-breathing) division of the 

 Gasteropoda. The sea-slugs, in which, like the 

 land slugs, the shell is absent or reduced, are 

 relatives 6f the land snails. Some of those found 

 on our own shores are handsome creatures, bril- 

 liantly coloured. Both groups fall under the divi- 

 sion Euthyneura, while the majority of the marine 

 univalves belong to the division Streptoneura (i.e. 

 Gasteropods with twisted nerves). The Gastero- 

 pods, in the course of the evolution of their shell, 

 have had the body thrown crooked by the burden 

 of carrying it; the Streptoneura are the forms in 

 which this crookedness is most pronounced; in 

 the Euthyneura it is less so. There are degrees 

 of crookedness even among the Streptoneura; 

 and the limpet is less crooked than the periwinkle 

 (see Table, p. 30). 



The older classifications of the Gasteropoda 

 were largely founded on the characters of the 

 shell ; but these, though in the main they hold 

 good, have required some modifications in recent 



