i6o 



PRACTICAL ZOOLOGY 



Sensitiveness to Surroundings. — The sense organs that make 

 the snail aware of the character of its surroundings, enabling it 

 to find food and escape its enemies, are situated on the head. 

 Unlike the mussel, the snail possesses a very distinct head. The 

 head bears two pairs of tentacles or " horns " (Fig. 89, FiL); the 

 upper, longer tentacles bear each an eye. These eyes, however, 

 are probably not organs of sight, but simply serve to distinguish 

 between lights of different intensities, or since snails are active 



« B 



9 if k 1 



Fig. 90. — The snout of a snail cut vertically and lengthwise to show the 

 mouth and rasping organ. 



1, dorsal wall of head ; 2, mouth; 3, jaw ; 4, radula ; 5, cartilage of tongue; 

 6. muscular wall of pharynx ; 7, muscles running from pharynx to ventral 

 wall of head ; 8, space in head for withdrawal of tongue ; 9, pocket for radula ; 

 10, oesophagus; 11, opening to salivary gland ; 12, fold behind radular pocket. 

 (From Lang. J 



at night, may be adapted to dim light. If touched, these tenta- 

 cles are quickly drawn in, being introverted like the fingers of a 

 glove. Food may be located at some distance, giving us reason 

 to think that snails have a sense of smell; the smaller pair of 

 tentacles is supposed to bear the olfactory organs. A third 

 sort of sense organs, a pair of statocysts, lie in the head and con- 

 trol the equilibration of the animal. 



Method of Feeding. — The particles that constitute the 



