MORPHOLOGY OP BRAIN AND SENSE ORGANS OP LIMULUS. 7 



details that might obscure the meaning of the broad facts I 

 wish to present. However, I shall describe the morphology 

 and physiology of the olfactory organ, and of the gustatory and 

 temperature organs on the appendages and elsewhere in detail. 

 In order to justify the comparisons instituted between these 

 sense organs and those of Vertebrates I have given a general 

 account of the structure and development of the brain and 

 median eyes. 



I do not hesitate to say that I believe the results herewith 

 presented prove beyond any reasonable doubt that the Verte- 

 brates are descended from the Arachnids. 



Part I. — Sense Organs. 



I. Gustatory Organs. 



A. Experiments on the Gustatory Organs of the 

 Mandibles. — If an adult horseshoe crab be placed on its back 

 with its abdomen hanging over the edge of the table, it makes 

 fruitless movements of the legs and abdomen to recover its natu- 

 ral position. The muscles, however, soon relax, and the animal 

 usually becomes perfectly quiet, except that after long intervals 

 the gills are raised and lowered a few times, and then held up 

 motionless a few seconds till every part, expanded to its full 

 extent, is thoroughly aerated ; they then sink slowly back to 

 their original position. If, while in the quiescent condition, the 

 jaw-like spurs or mandibles (PL 1, fig. 3,o. m^.)at the base of the 

 legs are gently rubbed with some hard object, such as a piece 

 of wood, glass, or iron ; or if water or air, 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 

 produced, usually none at all. But if a very small piece of 

 clam or other edible substance be rubbed ever so gently over 

 the stout spines that arm the mandibles, very characteristic 

 chewing movements are immediately produced. If all the 

 mandibles are touched in this way, or even moistened with a 

 few drops of water in which pieces of clam have been soaked, 



