AND PHYSIOLOGY. 



405 



ments of sensation. 

 " For just as a blind 

 man judges of the 

 proximity and char- 

 acter of objects by 

 the impressions com- 

 municated to his hand 

 by the contact of his 

 cane, with which he 

 examines them, so 

 may an insect or 

 crustacean receive 

 sensory impressions 

 from the nerves dis- 

 tributed to the basal joints of their long antennae, although 

 the organs themselves may be as insensible (or rather as un- 

 impressible) as the stick." 



746. Touch of Molluscs. — The sense of touch in this 

 group is well developed. In some of the orders the organs 

 are from two to four contractile tentacles situated upon the 

 head, or the anterior part of the back." u With some Gas- 

 teropods these tentacles are hollow and button-like at their 

 extremity/ and can be inverted like the finger of a glove." 



747. Touch of Radiates. — "The sense of touch is well 

 developed among the Echinoderms." It exists here and 

 among other Radiates in tentacles or feelers. Besides this, 

 other individual Radiates seem to possess a sensitiveness of 

 the whole surface of the body, though nerves can not be the 

 agents which secure this sense to them. 



Is it by an active touch that this sensation is gained by articulates ? 746. Describe 

 touch in molluscs. 74T. What is the touch of radiates ? 



