112 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS. 



structure to the ones we are about to describe occur in other parts of the head 

 and trunk, but stimulation of them does not produce any recognizable reflex. 



a. Reactions to Stimulation. — If an adult Limulus be placed on its back, it 

 soon becomes quiet, except that after long intervals the gills are raised and lowered 

 a few times. If, during the quiescent condition, the jaw-like spurs, or mandibles, 

 at the base of the legs are gently rubbed with some hard object, such as a piece 

 or wood, glass, or iron; or if water the temperature of the surrounding medium be 

 gently poured over them; or if the animal be vigorously fanned, or loud noises 

 be made near it, only slight, aimless movements of the legs or abdomen are pro- 

 duced; usually none at all. But if a very small piece of clam, not more than two 

 or three millimeters long, is gently laid on the surface, say of the third mandible 

 on the left side, care being taken not to touch any other parts, that leg will be 

 repeatedly raised and the tip bent toward the mouth, while its mandible will 

 move back and forth, alternating with the leg movement. Meantime all the other 

 mandibles and appendages are motionless. One may start in this way one append- 

 age after the other (except the ffrst and last pairs), until all of them, first on one side 

 and then on the other, are in action. 



If all the jaws are stimulated with food at the same time the normal chewing 

 reaction takes place as follows : The second and fourth pairs of mandibles move 

 in unison inward toward the median plane, and downward toward the mouth; 

 then back again in the reverse order. When they are farthest from the mouth 

 the corresponding legs (except the second pair in both males and females) are 

 quickly raised, flexed, and the tips carried toward the mouth, where they remain 

 an instant, and then fall back on to the under side of the carapace; the corre- 

 sponding jaw movement then begins again. The third and fifth pairs of append- 

 ages and the corresponding jaws work in unison in the same manner, but they 

 alternate with those of the second and fourth. At intervals these movements 

 cease, the abdomen is raised, and the stout crushing mandibles on the sixth pair 

 of appendages, which*have heretofore remained motionless, are slowly closed with 

 great force, as though to crush some object too large to be swallowed whole, or 

 to kill some struggling prey. These powerful jaws then slowly relax their con- 

 vulsive grasp, and the chewing movements are resumed. 



All these movements go on with the greatest precision and regularity, so that 

 the food that was placed on the jaws is forced into the mouth and down the 

 oesophagus. 



A drop of clam water is sufficient to start the whole reaction, which is per- 

 formed in the same manner as during the actual process of eating. 



If wads of blotting paper are used, wet with ammonia or picric acid, the 

 chewing movements are reversed, and the offensive object may be snapped up by 

 the cheliceras and rejected. 



Strong smelling food held close to the mouth, or to the jaws, produces no 

 effect, although chewing movements are instantly produced when the jaws are 

 touched by it. 



