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 antenne 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 (Zretmophorus) 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 
