42 HABITS AND INSTINCTS OF ANIMALS. CHAP. II. 



pluck them from the rock to which they were fixed, 

 they instantly shrunk hack into their respective cavi- 

 ties. 



(52.) Among the shelly Mollusca, the organs of sen- 

 sation first began to show a higher state of development. 

 The animal of the Solen, or razor shell, like all the 

 headless or acephalous Mollusca, is without any visible 

 head or eyes ; and yet, on the slightest touch of the 

 sea sand around the spot in which it is always buried, 

 it withdraws its long fleshy tubes into its shell, an I 

 sinks to the lowest depth of its cavity. The earth- 

 worm is no less alive to the least vibration of the ele- 

 ment in which it lives. On mild damp evenings, during 

 the greater part of the year, after sunset, these animals 

 may be seen in gardens, protruding so far from their 

 holes, that it would, at first, seem that they had actually 

 quitted them : at such times they appear stretched at 

 full length, and motionless, as if they were basking, 

 not in the sun,, but in the dew. If the observer walk 

 very softly, he may approach within a foot or two of 

 the worms, without disturbing them ; his ordinary tread 

 will make those nearest him disappear almost instanta- 

 neously ; but, if he stamp hard upon the ground, the 

 same effect will be produced upon all those that are 

 on the surface for the distance of fifteen or twenty feet 

 from him. In the cephalous Mollusca we first begin to 



discern the vestige of eyes, as in the slug and the snail ; 

 for we consider it beyond doubt that the black points 



