28 



NERVOUS SYSTEM AND SENSE-ORGANS 



surroundings, and help to detect the presence of food, or to give 

 warning of danger. Such are the tentacles of Jelly- Fish and 

 Sea-Anemones, the slender outgrowths on the head of a Sea- 

 Centipede, the two pairs of antennae on the head of a Cray- 

 fish, the single pair on the head of an Insect, and the tentacles 

 on the heads of Snails and Slugs. The " whiskers " of a Cat or 

 Rabbit belong to the same class of structures. They are stiff 



Fig. 1034. A Deep-Sea Fish (Eretinophorus^ with its Pelvic Fins drawn out into long Tactile Organs 



hairs, at the base of each of which a touch-corpuscle is to be 

 found. Such organs of active touch may either from the first 

 have done duty as sensory organs, or may have originally been 

 evolved in the interests of some other function. The former 

 is probably true for the feelers of a Sea-Centipede or Insect, 

 but the large feelers of a Crayfish (and very likely the small 

 ones too) were probably jaws at an earlier stage, having later 

 on been shifted in front of the mouth, and modified in shape 

 and structure to do duty as sense-organs. There can be no 

 doubt that the paired fins of Fishes were originally evolved in 



