MORPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS. 83 



ever, we find exactly the same nerves and the same ganglia, 

 but they are connected with permanent organs of taste. 

 Moreover some of these sense organs are single cells, strangely 

 modified, and recalling the isolated gustatory, olfactory, and so- 

 called *' tactile '' cells so widely distributed in Vertebrates ; 

 others are sensory buds, practically identical with those scat- 

 tered so uniformly over the body, as well as with those in the 

 olfactory organ. 



There certainly can be no doubt that the great masses of 

 sense organs in the jaws of Limulus have disappeared in Scor- 

 pions, leaving nothing but an enormous ganglion and nerve to 

 indicate their former existence. This fact is of special in- 

 terest in this connection, for it is exactly what Beard sup- 

 posed had taken place in the supra-branchial sense organs of 

 Vertebrates. 



In the free swimming merostomata-like ancestors of the Ver- 

 tebrates, we may assume that a change like that in the Scorpion 

 has taken place. With the transformation of the appendages 

 into gill arches the endopodites or the gustatory spurs of the 

 walking appendages were probably reduced to mere sensory 

 patches, appearing in the ontogeny of Vertebrates as transitory 

 gangliogenic thickenings of the ectoderm, or supra-branchial 

 sense organs. I have already pointed out that in Scorpions 

 {" Origin of Vertebrates from Arachnids ") the coxal sense 

 organs, ganglia, and nerve, were like the supra-branchial sense 

 organs of Vertebrates in their development and distribution and 

 relations to the rest of the nervous system. Now we can 

 show, in addition to this, that in Limulus the mandibles may be 

 regarded as segmental gustatory organs. They are centres 

 composed of aggregations of gustatory buds and cells, and 

 from them, or near them, may arise diffusely distributed sense 

 buds. The segmental aggregates and the diffuse organs in 

 Limulus are largely gustatory, and the same is in all pro- 

 bability true of these organs in Vertebrates, various theories 

 to the contrary notwithstanding. 



antennae of Insects is really an appendage ; it may be either a sense organ, or 

 the endopodite or exopodite of the cheliceral segment. 



