MORPHOLOGY OF BRAIN AND SENSE ORGANS OE LIMULUS. 35 



IV. Temperature Sense. 



The whole body of Limulus is very sensitive to changes 

 of temperature. This may be easily demonstrated in the 

 following manner : — If a crab be placed on its back and 

 allowed to become perfectly quiet, one may grasp the appen- 

 dages or mandibles with forceps and gently move them about 

 without arousing the animal; or one may touch the upper or 

 lower surface of the carapace, or the gills, with any object the 

 same temperature as the air, but the instant one touches any 

 of these parts with the fingers, or drops water on them warmer 

 or colder than the surrounding air, the animal at once becomes 

 more or less agitated, and moves the appendages and abdomen 

 about in vain efforts to regain its normal position. There is 

 no way in which we can make the quiescent animals start more 

 quickly or violently than by very gently breathing on the gills 

 and under surface of the body, although quite violent fanning 

 may produce no effect at all. 



If, holding the head within about a foot of a crab, one blows 

 little puffs of warm air on the parts about the mouth, the 

 chelicerse will snap with every puff. If the puffs of warm air 

 are made a little stronger the chelaria are brought forward, 

 and the chelae of the first and third pairs of appendages close at 

 every puff; tiie chelae of the second pair meantime, in both 

 males and females, remain motionless. 



Whenever the sides of the cephalo-thorax of a quiescent crab 

 are touched with the fingers prompt movements of the appen- 

 dages follow, the legs opposite to the point of contact, and on 

 the same side that was touched, beginning first. 



The gentle warmth of the hands held within two or three 

 inches of the sides of the cephalo-thorax, or from the face or 

 body when watching closely the experiments, usually produces 

 uneasy movements in crabs that were before perfectly quiet. 



The temperature sense is very acute on the lateral margins 

 of the thorax and abdomen, and on the tips of the legs, and of 

 the abdominal appendages, being apparently most acute inth^ 



